Swamp/Jungle would be good.
Caves/Caverns might be a good idea (They're not dungeons, but they're close)
Rules for entire areal games could be interesting (IE, above the clouds style)
Mountains aren't covered directly, though races of stone does help with that.
City environments could be an interesting twist. Same with dungeons, but they've been done sorta.
Extra Planar? (Hasn't this been already done?)