I, for one, am excited for more content or an expansion for Zombasite. I will think more about what I want from an expansion and post again in a few days. The most common critique I see is that the Zombie part of Zombasite feels tacked on, or feels like a sort of novelty. As far as I can tell from the Steam forums, most players who regularly play the game tend to turn the Zombie stuff off after the novelty has worn off. Long term play tends to make the Zombie stuff more of an annoyance than a fun feature or game play mechanic. By turning the Zombie stuff off, players are basically turning the game into Depths of Peril 2.
It is hard to say whether people turning the Zombie stuff off means that you should use the expansion to integrate the Zombie stuff deeper into the game so that it matters more, or whether you should use the expansion to focus on the non-Zombie parts of the game (i.e. the meat and potatoes ARPG mechanics) that keeps the core audience coming back. I would be surprised if an expansion focused purely on additional Zombie mechanics was met with enthusiasm. Fleshing out town management or the ARPG side of the game might be a more solid bet. One thing I have noticed is that the dungeons in Zombasite are really fun, but the player doesn't spend a ton of time in them. Maybe you could add some sort of dungeon mode that would let the player play the game as though it were sort of a Din's Curse 2 (the way turning off Zombies makes the game a sort of Depths of Peril 2).
I am sorry to hear it hasn't sold as well as you would like, but maybe the GOG launch will help that. Also, it seems like most (if not all) mid to small size indies are struggling. Given the poor condition of the mid to small size indie market, I suspect Zombasite is faring better than games from studios of a similar size to Soldak's.
Concerning the new game, it sounds like a promising idea. Have you played Rogue Legacy? Also (and this is further afield) Crusader Kings. These might provide some inspiration.