Ok so, I think plugins SHOULD be allowed to create new worlds, BUT, it would likely have the same problem as spigot plugins that do it: they can time out the server. Yes, the plugins generate outside of the world tick thread, but if the generation takes up too much processing power or memory, the server can crash (I'm speaking from experience here, it's not hard to crash a server with world gen. Try loading a Citiworld world on a server with <2 gigs ram using Multiverse. It either takes 1-4 minutes or takes everything down with it).
So long as a plugin can load an exact copy of the overworld by using the same speed without crashing the server, it should be fine. Doesn't matter if a custom gen crashes it, that's the server owner's problem.
So long as a plugin can load an exact copy of the overworld by using the same speed without crashing the server, it should be fine. Doesn't matter if a custom gen crashes it, that's the server owner's problem.