Maybe some type of block that naturally floats about, like a kind of cloud that you can build on. It'd allow you to build floating islands, airships, and whatnot, that would actually float around the world. Then perhaps some type of propeller type craftable item so you can actually steer your skyland/airship/enterprise/whatever the eff you've made, around.
Also, like has been said, some changes so that iron ore will spawn together in a higher concentration, or, at the very least, a new item that can point you towards iron or diamond or whatever. Maybe something like the compass, except instead of redstone you use whatever element you're trying to make it find.
Personally, I'd really love to see some NPC's running about. Like, for instance, you spend a day or more making a nice little town, and you get to attract people to it, who will then interact with one another, having kids and whatnot. Town gets populated, and you actually have a use for that giant castle of yours, as you are now king/queen. I'd love to see it get real crazy though, with you being able to order people about to become miners, guards, item crafters, or something like that. Barring that, maybe they just do it on their own without you getting to decide who does what.
Even better, and I know that this is probably kind of impossible, make it possible to mix the game with something like Sim City or Civilization, where you can make blueprints and build specialized buildings, like colloseums or stables, and be able to instruct your minions to make them for you if need be. Multiplayer games become epic in scale, as each person gathers up and builds their own cities, raises armies, and goes into massive scale combat with one another.
Not going to happen, but I can dream.