As for underground tilesets, I already have planned three variant Underdark settings. No guarantee they will get done this year, but they won't be as hard to do as this one, since many of the parts will be either reusable (from here), or simpler to construct.
The first will be karst caves with a lot of limestone decor. It will feature both wet and dry terrains, including flowing water, shallow water very much like that in the Dwarven Forge Water Cavern Pack, as well as a 5m height transition with smoothing crossers similar to this set (no need for 10m, IMO). This cave will also feature bottomless chasms. Natural cave entrances in the mountain tileset may lead to this type of cave system, especially once I get the limestone outcrop terrain added. I'd like to be able to transition from natural caves to deep crypts. I've got a lot of sketches prepped for room groups for this tileset.
The second will be more igneous, having far fewer columnar features, and a lot more mushrooms and crystal features. In addition to water and chasm, you will also see lava and lava crossers. Many of the mine and cave entrances from the mountain set will lead to this cave type. I'd like to be able to easily transition from this cave system to Dwarven halls. Expect the crystal features to be over-sized and almost ridiculous, like in dungeon siege. Also expect newer appearances like razor rock, which I will probably portray as jagged obsidian. For the crystal-less areas, I'd like to build some placeables to go over the walls, so you can have players mine simple gems and ores in the dwarven tunnels. I feel this cave set should feature a TON of room-like groups. I probably won't personally do any Dwarven halls remake, since we have a good one already. It also already features most of the stuff I was going to steal from dungeon siege.
The third will be utterly creepy with lots of unnatural shadow added. The rock itself will either absorb light, or glow, depending on the terrain used. It will contain huge quantities of odd mushrooms and other glowing stuff. In addition to lava, acid and other strange liquids will replace water features. I'd like to make a lot of earth-node and Faerzress-infused groups. The goal is to be able to transition from here to demon prisons, similar to the Dwarven halls, but with a much older and darker feel. Also expect an entire terrain which is just mushrooms and slime from wall to wall.