As the Groove Generator seems to be the key selling point for a lot of us (you've instantly got a whole market of people who loved GA2 and haven't found a replacement since), I would suggest finding a way to allow people to really automate and record what they do with it.
Here's the use case I want. Let's say I've created a song structure in my DAW of ABABCB. I'll find a set of grooves in RealiDrums for A, B, and C, which I can map to keyswitches to move through the grooves during playback. Then, I'd map each of the complexity sliders to knobs on my external MIDI controllers and record the automation. As the song plays back, I can twist and turn the knobs in real-time to adjust the "performance" of the drummer to follow the dynamics and feel of the song. So, for example, if I want to build up to the chorus, I could slowly twist the complexity of the hats and snare to humanize the dynamics. When I'm happy, I can print the MIDI to save the performance (in case i want to swap out the snare for one in another library, for example).
If you really want a killer app, I'd suggest allowing people to import their own MIDI grooves, and finding an algorithm to intelligently add or remove complexity from the original groove. The results may not be perfect, but given that we're working with mathematical subdivisions, I think one could create a heuristic that would work for most standard beats. I'm a dev myself so PM me if you want me to do the legwork to find if there are any existing algorithms out there.
This would be uber-useful and would probably allow me to throw away virtually all my other drum libs, and I suspect others would feel the same way.

