A Build-your-own Dungeon DLC. It can be above ground. I am thinking traps, triggers, fight “boxes” (where NPCs or Pets will fight player without being removed from game themselves,) and treasure chest that will fill with random items.
The siege weapons that were discussed a good time passing. Something even single-players can enjoy.
Separate boss scaling slider for Solo/Co-op players and private servers.
Domestic placeables that will make your home base/camp look more like a place being used. (The assets from the NPC camps would be a good start. Food racks, single anvil, a spider pot that will sit over the fire, tableware (okay with me if it comes as a single item), garden items that can be planted and harvested outside the boxes, placeable/non-harvest-able plants…
A floating platform for crossing water (some pets won’t do it others get lost or hung up at the bottom.) Think elevator style but instead of the elevator there is a floating platform that won’t allow pets to fall through. (Ferry style)
Replay / new player content: More choices in T1 and T2 structures, low level tent (like the green one in game,) stone weapons that have a regen bonus (for that difficult transition between stone and iron…) When I player chooses to reroll, there could be an incentive or added options without giving them an in-game advantage.
A dungeon respawn map so one does not have to rerun the entire dungeon if defeated in the boss chamber. (It can be a part of the dungeon where a player can choose to be respawned at that point rather than to their bed.)