I would like to see more small underground biomes. Right now the variation underground is quite limited. There are a few different layers, each having a handful of mobs, many of them palette-swapped slimes. I don't mean to complain about this (and killing slimes is always a good video game staple), but I feel like there's a treasure trove of potential waiting to be unlocked. Rather than adding more endgame material or super-outlandish space biomes (not that that wouldn't be cool, too) or whatever, why not add some texture to the depths of Terraria? Think of the underground wooden structures that can now be found (and that I completely love- thanks Re-Logic!). Now imagine that there are 5 or 6 different varieties of things like that, but quite a bit bigger and more complex, each with 2 or 3 unique mobs. I guess what I'm longing for is a little more of that Metroidvania flavor, where after traveling through the labyrinthine world a bit you start to remember incidental landmarks and it all starts to seem like a real location rather than a huge block of self-similar dirt and air pockets. Handful of examples: Goblin mines, sunken gardens w/ trees, buried castle ruins, ice caves, crystal caves, fossil deposit, you get the idea. Forgive me if all this is just completely obvious and it's a given that eventually something like this will be added. I just want the world below the surface to be a strange and wonderful adventure.