I think the ability to enter new, physical planes inhabited by magical beings or something of the sort as a late-game thing would enormously expand the playability of the game, and a player's motivation to continue forward. I'd love to see some planes that are hellish and require late-game and others that are just magical and maybe require luck or adventuring to get to. Just my wishful thinking...
I myself would love to see atleast one type of plane that sort of goes the "eldritch, surreal horror" way. Like a "land of confusion" where nothing makes sense - bizarre, completely randomly generated creatures (even weirder than already existing titans or FBs) that aren't neccessarily hostile and could be adopted by dwarves, and strange plants, i.e fleshy organic trees that sprout giant bubble-like fruit with elephants or other animals inside them, etc. Potentially a wandering "deity" megabeast that holds the dimension together.
Or alternatively something like the aforemention Pure Zones from OFF where it's just pristine nothingness - white nondescript grass, trees and stone, remnants of buildings and such - no living beings aside from giant demonic entities that'd be the "megabeasts" of the dimension.
In any case that's probably veering too close to "suggestion" territory.