You can always have some kind of fantastic environment. And by that, I mean, dumping the idea of a giant sphere as the world. Theres always the idea of a giant tree or an earth within an earth (which falls into the vertical stacking mentality). Or theres having the world on the inside walls of a sphere. Then theres relativistic effects, where you could have a smaller plane of existance where distances on it are shorter, but move the player back to the normal plane as though he travelled increadible distances in a short time.