Firstly thanks for the great emulators you guys are building.
Are we able to suggest emulators we would like to see added to run on RetroArch?
Yes. Feedback from users is always appreciated and encouraged. This is how the entire open-source community continues to work, after all. 
There are a few limiting factors, though, such as the requirement for software rendering (i.e., no 3D-accelerated renderers, such as current N64 emulators, PCSX2, Dolphin, etc) and some internal serialization stuff. One of the actual devs would be able to explain more.
It would be good if Retroarch had cores for some old computers, maybe a port of MESS would do all of these:-
Amstrad CPC ZX Spectrum Atari ST Amiga
1000’s & 1000’s of old school games could be played again