What character build would you like to play but can't due to setting and tone limitations?
For example Necromancers: these are pretty universally evil and their abilities are pretty disturbing so in many homebrew campaigns I imagine it would be hard to make them work.
Or maybe you have a specific combo in mind, but it requires multiple players cooperating heavily.
Or it's just way too high level. Or the PC is just evil. You get the idea.
Edit: I know that I can play a necromancer in the official setting even if it still has problems IMO. But what about any other high fantasy, be it an established setting or a homebrew one?