A couple things I would really like to see:
1. On the topic of underground rivers, I think it would be awesome for them to be places as normal rivers, just underground. The river could even pop up aboveground for a while, then go back under. This would be more realistic than water falling in some random chasm, and more interesting as well.
2. Yes, we need more "uber" materials than adamantium, but there's the fact that these materials shouldn't be just as good as what adamantium does best: make rockin' armor and weapons that are really lightweight. If more endgame materials are added, then they should be both a) really good at doing something adam. doesn't, and b) be accompanied by suitable challenges. For instance:
Leviathan Blood: A rare material that is formed when a terrible creature dies in the Ocean. Extremely strong out of water, but workable under-water.
Fire Tree Wood: A type of tree that can withstand a surprising amount of heat, withstanding the heat of magma even, partly due to its density. While its heavy, and hard to work with, it has the most mysterious property of being able to fuse with metals.
Swamp Heart: A material sometimes found in the soil under a swamp, where various plant and animal matter has seeped into. Looks and tastes disgusting, but when brewed, makes a drink so wonderful that those who drink it are imbued with the courage and valor of 10 dwarfs, and can perform superdwarven acts.
Anyways, what I really want in this game is huge elaborate dungeons to explore that are created in world-gen.