DBZ has such an immersive universe: http://www.zerochan.net/
First, when I try to recall the whole journey I feel like DBZ had a few too many unnecessary enemies and story arcs. So we should try to narrow these down. When is it time to end an enemy? Did Broly really need to return twice? I really disliked Bio-Broly as a movie, the other two were acceptable as Broly was Broly, the legendary Super-Saiyan, not some pile of goo. Likewise, is Cooler's return as a regenerating robot thing really acceptable when Freiza's return was so lacklustre? Or when is it right that an enemy is just created used ideas that other enemies had? Super Android 13's ultimate form is somewhat like Cell's rise to Perfect Form and did not feel satisfying. One of the problems with DBZ and Sounen anime in general (in the case of not overdoing it) is having good, well thought out enemies that can reasonable fit into previous canon. I would accept Broly, because he was kick-ass and because he fits into the whole prophecy about Freiza being defeated by a Super Saiyan, but when he is turned into goo it just gets a tad silly. Buu could reasonably rejected as he is a fighter of legend but no one mentions him in previous arcs before he turns up. Likewise Cooler is not mentioned by Freiza or King Cold. However, I would say these backstories need to be well fit into the canon. Bojack, Super Android 13 and Cooler just seemed to be tacked onto the main series. Bojack (rather conveniently) is released when Cell destroys King Kai's planet, Super Android 13 is just another Android with the same sort of powers and Cooler is just the unmentioned brother of Freiza. All these enemies just feel tacked onto the main story as lame excuses to have more fights and make more money rather than say Broly, who was more intricately worked into the main storyline.
These are only the memorable enemies though, other ones I would take out, purely as unmemorable: Garlic Jr (from Dead Zone and the main series after Freiza but before Cell), Dr Wheelo (from the World's Strongest), Lord Slug (from the movie of the same name) and really any movie enemy before Cooler (so the guys from the Tree of Might too). This is purely because they felt (and looked) more like Dragonball enemies, rather than Dragon Ball Z enemies. When I think of DBZ I think of DragonBall I think more of Earth based enemies. I suppose everyone has a different opinion on who is a worthy villian and who is not, but my pickings are very slim I suppose in hindsight. However, there are other issues that DBZ has not to do with the just amount and varying quality of its various antagonists. Another big issue this anime has (because of the numerous every stronger enemies it must use to keep the series going) is the forms that the protagonists must take.
These forms the protagonists must take, like the backstories of returning and new enemies, get more and more farfetched as the enemies get better and better. Saiyans, as the protagonists of the series, got more forms that were stronger and stronger in order to deal with these more powerful villains. Throughout DragonBall and the start of DBZ there is the Great Ape, but this is acceptable, Saiyans are aliens after all, so I do not have a problem with this. Nor do I have a problem with the idea of a Super Saiyan, as in order to beat Freiza, the ultimate evil, Goku had to become this, so it is again acceptable. What I don’t really accept is how many forms and tricks a Saiyan has beyond this. I admit I do like the idea of the Ascended Super Saiyan, where is it not another form but Saiyans just pushing themselves and their limits. This results in the speed vs raw power question which was cool to see. However, when Super Saiyans have other levels it really just gets a bit silly. With SSJ3 and SSJ4 (in DBGT) it kind of gets farcical. Most of all I was motivated to make this sort of post after seeing what I thought was the most ridiculous thing ever: the Super Saiyan God. I mean fusing was perhaps a cool way to make a more powerful fighter but when they can turn into a God, it gets a bit over the top. I mean it was like they couldn't make SSJ5 for DBZ because it only happens in DBGT and this would break the universe so that made the Super Saiyan God. It was definitely way over the line, for me anyway.
The real problem when we get down to it with the different forms for our heroes is not believability. These are aliens who can do strange things and basically have superpowers. All believability in terms of our reality is thrown out the window. Plausibility within an established universe is what I am talking about. Yes one form of Super Saiyan is definitely plausible, because there were legends, ones that other characters (namely Freiza) believed. However, the legends said that one had not come about for over a thousand years. The universe that had been established showed that being a Super Saiyan could be achieved but it was a very hard task. So due to how hard it is, is it really acceptable that those other than Goku can do it, and at such a young age in the case of Trunks and Goten? And is it really plausible that even when it was so hard to become a Super Saiyan that there are multiple unseen forms of it? I can accept within the universe that there is one form of Super Saiyan but not really two or more forms, or that anyone other than Goku and maybe Gohan could transform into it. Also it is believable that Namekians can fuse (they do it numerous times), but I don’t think Saiyans should be able to do so (this is more based on personal preference). Likewise I think it is plausible to have these omnipotent God characters (like in the newest movie) but I don’t think that Super Saiyans should be able to create one on the spot, no matter how mystical the ceremony may be. So in terms of forms I don’t find anything beyond Goku and Gohan really going beyond Ascended Super Saiyan as truly plausible within the universe. I mean if we look at Freiza’s, Cell’s and Buu’s various forms they are all given reasons. Freiza had to change many times to suppress his power, Cell got a new form as he absorbed the Androids and got closer to “perfection” and Buu got his new forms from just eating different opponents. The Super Saiyans forms are just them getting angrier and stronger really, not a great explanation to be honest.
Just as the forms our heroes take to defeat increase and get more ridiculous so do the power levels for both heroes and enemies. Now power levels, or ki, didn’t have to be a thing for the DragonBall universe but they were established and measured with the scouters Raditz, Vegeta and all of Freiza’s minions had them. Originally ki levels did a good job of showing how much of an obstacle our heroes had to overcome and how much they had grown in strength. That being said as it goes on these get out of control as more and more enemies are established and these enemies must be more powerful than previous ones. Enemies must be stronger in order to keep the series interesting (obviously). This is alright, however as soon as there are numbers brought into things it starts getting difficult. Without numbers, everything gets a bit better as we don’t feel as much of a need to compare the relative strengths of everyone over the course of their journeys. At the start of the series, Goku had a power level of perhaps 1000 in the Saiyan saga (as he couldn’t beat the 1200 Raditz alone), is then calculated to have power level of 40,000,000 even without turning Super Saiyan. I know he learnt many new training techniques (such as training under high gravity) but could Goku really grow 40,000 times as strong?
Let’s take another example. Krillin during the Saiyan saga is just over 200 but is then calculated during some of the later movies to have a power of 3,000,000, if not more. Is it really plausible that someone could increase their strength by a multitude of fifteen thousand? I don’t really think so, even within a universe where you can create your own energy (I mean he is fully grown throughout it). I fully understand that the later power levels are pure speculation by fans and those who wish to rank and compare enemies. Yes they are not canon, at least (not beyond the Freiza saga) but when a universe establishes a set of rules, like ki levels being measurable by numbers, then it must be able to stick by those rules. However, in the case of DBZ, something had to give, as power levels, it seems, were too out of control and so these things should be considered beforehand. Before I wrap things up I would like to mention that One Piece, like DragonBall Z, gives its characters a number ranking (with bounties) but it is a bit more plausible. This is because, rather than being based solely on their power, it is based on what they have done and it is not grown by ten-thousand-fold.
Someone’s cool product design for scouters: http://www.deviantart.com/art/Scouter-Product-Display-200429685
In terms of story arcs, I would say that the destruction of Cell would be my limit. Although Cell was a great enemy (some might say the perfect one), his back story is very convulutated with the timetravel, Android absorbion and just creation in general was a little improbable. You might say this should eliminate Cell but I think Cell and the Androids. However, although he was Earth based, he was more of an evolution of a DragonBall character (with Dr. Gero and the Red Ribbon army) than say Garlic Jr was and so he seems acceptable. Likewise the story telling and characterisation within this arc seems good with it ending as Gohan unlocks his potential and takes over the role of his father, after Goku sacrifices himself. Also the build-up enemies (the Androids and Cell Jrs) do all have their place in the story and it all seems fine enough. Although the Super Sayian form of SSJ2 is a bit of a cop out I suppose I could live with it. Buu on the otherhand feels over the top. Although he is the most fearsome enemy in the universe, he is no mentioned beforehand. Cell cannot be mentioned beforehand because he was being created in a laboratory. Likewise, Buu's build-up enemies feel not so well worked into the plot (with the multifloored base of Babidi's hideout), and the many forms of Buu just feel tiresome after a while (as all he does is lose and then absorb the next Z warrior). Likewise the forms of the Z warriors start to feel overdone, as here is where fusion, Gohan's ultimate form and SSJ3 come into play. Also this story represents a passing of the torch from Goku to Gohan. Gohan has unlocked his hidden potential (not really if we include the Buu story) and is ready to protect the world. With the end of Buu yes we have Uub but he is a new character entirely so it seems out of the woodwork, like most things in the Buu arc. So I think Buu goes too far but Cell could be my limit.
However, this being said in short and to be honest anything beyond Freiza on Namek gets a bit farfetched really and he should have been the main antagonist, with Broly as an after series movie. This should have been the end of it. Freiza was seemingly the ultimate hill to overcome, the one who could only be beaten by the first Super Saiyan. This was it the ultimate end, when Goku ascended above all else who had tried and saved the Galaxy (not just the Earth). This is when the "Z" warriors became interplanetary heroes, not just ones for Earth. This is also the point at which Goku, the young boy we followed from Dragon Ball, became the best fighter in the universe. His journey and thus our story is over. When it comes to other aliens like Buu, (and from the movies) Bojack, Cooler, they don't feel as hyped up as Freiza and are thus easy to cast out. We never hear about them until they turn up but Freiza was sort of the ultimate boss of the Saiyans who invaded Earth. In terms of numbers (a problem that plagues long running Shounen) Freiza was a nice round seemingly unobtainable number of one million. Once we get beyond Freiza the numbers certainly blow out of portion, considering one of the most famous lines was "over 9000" (over 8000 in the Japanese original). The Freiza arc also answers many questions about the mysterious dragonballs themselves, which were not answered previously, such as where they were from. Admittedly there is no real redemption for Vegeta and thus no Trunks etc but it doesn’t matter as the story is over.
Many think that Akira Toriyama originally intended to end the entire Dragonball story with the conclusion of the Freiza saga but because the series was so popular that he was more or less "forced" to work on it. This maybe true maybe just rumour I feel like I have laid out my reasons for this theory. In conclusion, for DBZ I think Freiza, with Broly (and perhaps the first Cooler) as an acceptable after series movie or two, should have been the end of the series. This being said I could accept Cell as well. But I think the latest movie with its "Super Sayian God" form just shows how seemingly farfetched DBZ, one of the most beloved series of all time, got before it ended. Series, like One Piece or Fairy Tail, need to end sometime and I think it is better to end it sooner than leave it hanging forever. Pokemon likewise feels this way. Arcs (movies or series based) begin to feel formulaic, the power "level" of enemies feels unnecessarily bloated compared to the early antagonists and only way to keep the heroes interesting is to beef up their powers with some more ridiculous form for them to take. Basically the problem is that as the world gets deeper and deeper it all gets less and less worthwhile as the feeling increasingly goes from one that thinks the story is progressing to one that it is stagnant and will never reach its goal. For me the best series end at the right time.
