Guild Wars 2

It would be really fun if we could design our own instances. If you really want to, you could have it as something similar to the current custom pvp maps with buying time and stuff, but the important thing is that much more can be customized:
able to edit terrain. there might be 30 different tiles and you can arrange any of those on a 10×10 grid in whatever combination you want. or maybe some preset ones that you can adjust such as making part of it taller or put at an angle, etc.
you could even pay x for a 10×10, y for a 15×15, z for a 20×20, etc.
able to add npc’s / enemies
special bundles/skills (probably just any skill that’s currently in the game – I wouldn’t go so far as to be able to make your own skill, but maybe be able to edit a current skill such as increase the range or decrease the cast time)
essentially I want a map editor
I imagine you could add your instance to a public roster if you choose so anyone could play on it.
people could make their own dungeon-type rpg’s, pvp with special rules (like capture the flag or hide and seek), [jumping] puzzles.
I would love to make a DOTA map :P