Since I believe Dragonborn will be the last DLC we will see for Skyrim before Bethesda begins work on the next Elder Scrolls game, I hope they release a few small freebie DLCs like they did for Oblivion, like maybe a ruined castle or a deep dungeon delve, bigger than anything we've seen so far. I would like to see Tamriel's version of the underdark, or Moria, a dungeon that spans for miles and miles complete with shops and inns and taverns and small cities. Perhaps we will find the lost dwemer people down there? Maybe there are vertical shafts and you need special climbing gear to traverse certain areas. Maybe there's a dragon down there that dwarfs any on the surface, different than anything we've ever seen before? Perhaps there is an underground jarl with his own underground hold and he can make you a thane and offer to sell you some land which you can build something completely fanstastic underground and that can be your new underground lair, lol. I would definately buy this if they made a dlc like this.

i wanna ride giant earth worms and form my own under kingdom with which to invade nirns core and kill all the planets inhabitants through rapid cooling

This are all fantastic ideas and I'm sure Bethesda is taking them into serious consideration as we speak.

Dark dungeons would be awesome, but the dwemer will probably never return.. after all they have been gone for like 5000-6000 years or something.

Beth did promise three mahor DLCs, so hopefully another like DB and DG will come out in the near future.