In 1.4 I would like to see:
One new stone type such a marble (white with light grey veins) which appears more rarely than normal stone, just so that digging underground isn't so boring and people don't have to use wool when making Greek-style buildings. It would also be interesting if a gem such as emeralds appeared more frequently in the new stone type.
I'd like to see villages refined to be less derpy and more useful (ie make it less likely for them to appear over huge holes or water, or in ways that bury/strand half the houses and make the villagers less likely to vanish randomly just when your village is getting interesting). I would like it to be a valid form of gameplay to adopt a village and pimp it out, and to get some benefit from doing that.
I like randomness in games - the chests in the tunnels and the new buildings are fun to expore, but I'd like there to be a risk/reward balance for opening these chests. A chance of spawning enemies around you when you open them would be entertaining. Would also be a suprise for people cheating to find goodies (would also be awesom if you could set up a trap chest full of mobs to get people who stick their noses where they shouldn't - would also make a fun game show lol)
I haven't really thought all that much about the future of vanilla Minecraft, as I've been writing ideas for a new 'flavour' of Minecraft in my head (it's a wee bit too complicated for a mod I think). It's sci-fi, but the main focus isn't on the theme, but rather providing a challenging multiplayer environment, where you can't just go outside and run around during they day, and equipment upgrades and co-operation between players make a huge difference. The biggest difference is that it is set on a planet with a non-breathable atmosphere, so all the delightful machinery you can make in varous mods suddenly has a purpose (beyond just making delightful machinery). You NEED power for your air scrubbers, you need to recharge your suits, you need to develop upgrades for your suits etc. I'm having fun coming up with new ideas and mobs and hope I have it in a form to share with people someday soon