I'll join the fray with another request for Drox Operative 2: Even Droxier.
One of my biggest problems with Zombasite is how clunky the character movement and combat feel, especially compared to something like Diablo 3 (yeah yeah I know AAA, millions of dollars blah blah blah) or even Torchlight 2. I have no problem with the graphics and engine, though I see where others are coming from.
Another thing that Zombasite had against it out of the gate was that it appeared to just be a mashup of Din's Curse and Depths of Peril without breaking much new ground. Yes, the engine updates and such were nice additions, but at a glance it APPEARS the same as before; same grid of maze-like square maps, same static towns, same classes and skills, etc. And those are legitimate complaints. But you're already starting from a disadvantage being small and "niche", and I think that this perception further chipped away at the potential market.
For New Game, I'd suggest to really focus on making movement and combat feel smooth and fast, and look the part. If you're continuing with your existing engine (and art assets), that may be a challenge, but the "feel" is so important. We know your systems behind the scenes are great, the Soldak Living World(tm) is great, and the complex interactions and choices that have far-reaching affects in the game world are great... but movement and combat feel, and streamlining and prettying up UIs would be my suggestions for top priorities. It sounds like New Game will be a little more straightforward than the majority of your catalog, so polishing up UIs shouldn't be too difficult (says the guy who doesn't have to implement it).
The last thing you want is for screenshots and trailers to come out and have everyone say "Hey it's just Zombasite with a Rogue Legacy mechanic tacked on, meh". It has to be different enough that people don't think you're just making a tweak to your existing games and throwing it out there with a new price tag.
(Personally, I think one of the reasons Drox is so loved has to do with the smoother movement, especially when you throw on the thrust. I never get that same feeling in the other games.)