Here's a few ideas I have for sound chips/systems:
AY-3-8910 (Used in the MSX)
To some people, the MSX sound system might sound similar to the Master System, but the MSX is FAR more capable in several ways. It has a greater range of notes and more capabilities in white noise generation. Plus, if this is added, those of us that REALLY want YM2608 support would be satisfied because it can still be replicated by combining the Genesis FM, Arcade PCM, and the MSX PSG.
YMF262 (Used in IBM PCs)
While similar to the now defunct YMU759 chip in a few ways, the YMU759 chip felt very limited in comparison to all the other chips. (From what I used of it at least) The only other OPL3 trackers in recent memory, such as AdLib Tracker, are stuck with using key commands only, so putting OPL3 support into DefleMask will increase accessibility to making that kind of music.
SNES, Amiga and Neo Geo can be easily simulated with FL Studio for me, and implementation in DefleMask seems much more difficult with these systems than the above ones. I don't know much about POKEY, but from what I've heard, I'm still much more interested in the above systems.