Or- A world of unnatural gravity, with floating islands, and cloud bridges between them. Sure the randomizing aspect of it would be.. hard to make, in which case, probably make something like elytra to help travel between each island. With a jump pad in each one.
Not sure about mobs. Flying ones...
Right. So first things first, how about, a world filled with Machinery, steampunk and all that.
Mobs: Possibly something along the lines of corrupted engineers, Flying Mini-bots, and a few stationary turrets, where you'd have to dodge their -timed- attacks.