the biggest things i want to add is bigger dungeons, more tamable mobs, and i personally would like a official aether. (but if the aether mod creator is reading this, (probably not) update to 1.3.2 please.) hey look, a parentheses in parentheses, back on topic, and also remove the no-clip creeper bug.
This might have already been said, but a pre-Adventure Update mode with the features of Beta 1.7.3 or so would be cool, and satisfy the people that hate the newer additions.
TT2000, you are genius.