Tougher monsters that only spawn deep underground seems like an obvious addition. Cave exploring is still my favorite part of the game, and fleshing out the dungeon-crawling "roguelike" aspects of the game would be awesome for those of us who love that part of the game. In fact, if we could run into proper "dungeons" underground, not just the single-room kind they have now, but manmade underground rooms and hallways filled with deadly monsters and treasure - that would be
awesome.
Edit: This is where I'd like to build next, on that crazy cliff jutting out to the left of the cursor.
That thing to the right of the cursor is neat too, but that'll come later. When you stand on top of that mountain on the left, you can walk through clouds. I built a stone tower in the center of the mountaintop, to measure the distance to the build ceiling, which you can see poking up out of the clouds on the left. My first thought was to make a castle on top of the cliff that juts way out to the right, but I won't be able to build it very tall. My second thought was to make a statue or something built into the cliff face, like one of them bitches on the front of a ship. That'll be a challenge for sure; I don't know of any way to build scaffolding from the top going down, so I'll have to build up from the ground below instead.