Quote Originally Posted by Max_Killjoy View Post
Yeah, I don't quite get the whole "we must go to other planes once we've hit X level" thing, though it seems to be largely a D&D thing IME. I especially don't get the appeal of it in context of D&D's standard clockwork extraplanar cosmology.
The idea of characters being too big for the world as it is so they need to go somewhere else now is actually quite common in zero-to-hero style stories. It's everywhere in comics, where character in Marvel and DC routinely 'graduate' from Earth-scale issues and go off to fight on a 'cosmic' scale. The recent Captain Marvel movie is a textbook case of this. It's also common in major shounen anime. Dragonball, classically, uses this trope in spades, to the point that by the later portions of super they're fighting over the fate of entire universes, but even in more grounded series its very common for the protagonists to go to some new country or region after they're 'leveled up.' This is also something you see in video games, including MMOs, which tend to have level-gated areas.

In terms of fiction-level balance, you can utilize this sort of 'leave the world' measure as a way to balance martial and caster power. If at the point where the martials cap out the casters can still grow massively but they have to go somewhere else or even just spend all their time worrying about otherworldly threats, this can be used to balance the game. Dr. Strange, for example, lives in New York but spends most of his adventuring time fighting weird extradimensional entities to the point that he's far too busy to interfere in Earthly affairs unless it's some Thanos-level crisis. Closer to D&D, in Nehwon, Ningauble and Sheelba both have powers that dwarf those of Fafhrd and the Gray Mouser and could snuff them easily, but they're far too busy playing weird wizard games across the dimensions to bother.

Unfortunately this sort of thing is hard to manage in a game because all it takes is one god-wizard who refuses to leave home or decides to come back and meddle to massively upset the apple cart. The character of Xaltotun of Python - the wizard who serves as the antagonist for the Conan novella Hour of the Dragon - is pretty much emblematic of how this can unfold.