Animals: maybe certain types. too many becomes a little unnecessary. I'm all for a muuuch raised mob count however!
Plants: palm tree yes. Past that I can't think of anything Minecraft really needs. I'd like to see an oasis added to deserts though.
Buildings: elaborate. I think we have enough blocks in new colors for now for most buildings, but since you mention wheels i assume you mean blocks meant to be decorated like furniture, etc. IMO part of Minecraft is having to build the things yourself out of the materials you're given. If you want total realism in your builds, play Brick Rigs or Gmod.
Old stuffs: again, certain things. Jungle temples could be much bigger. Certain things that are even in the game now or previously could come back or have a broken version of themselves spawn. That one's something to mess around with