Games are generating many times more money on iOS/Android then any other type of apps. Why ignore such a huge industry?
Today we clearly can see shift to midcore/hardcore mobile games like MMO Real Time Strategy.
This genre means user is forced to operate with a very complex system whith very high cognitive load.
This games age very heavy on UI. Basically 90% of such games is UI.
For example our upcoming sci-fi title consists of 162 unique screens with many states.
We need a tool which will ease the process of prototyping/designing/iterating/approving the screens using our existing UIKit (a huge set of pngs).

So the basic and really simple features which will make a whole industry (!) rush into your product are:
1. Place linked (not import) *.png
2. Setup 9-slice-scaling for png by entering 4 numbers- top,bottom,left,right. (THIS WAS A VERY HUGE DEAL when Axure introduced this feature)
3. Ability to multiply imported\placed png by custom rgb color. This is super important because most of UI-heavy games use black-and-white sprites for buttons/panels so it could be reused. Like we do not have to store GreenBtn.png BlueBtn.png etc. We just use a veeeery simple line in shader to multiply each pixel value by needed color:
c.rgb *= multcol.rgb;
I believe this can be done in XD like colorize feature, This is important because nobody watns to maintain two sets of graphics bw and colored(for prototyping)
4. Export a list of all elements with its names (or placed file names) into any machine readable format like JSON screen by screen
5. Only two blending modes: Normal and Additive.

I believe thats all you need to become leading (one and only) tool for games UI/UX prototyping.