I wish that deserts were a bit more... full. Yea, there are cacti, villages, the occasional temple and well, but seriously. I think that adding SOMETHING, idk what, would be a great addition for Minecraft in general. Maybe some sort of rattle snake, or a new boss.
Other biomes in need of more stuff(IMO) are:
Savanas
Taigas
Swamps
Jungle Edge*
*It's so rare... maybe an achievement? "Edge of the...Swamp?"