If you set up a development environment and go through some of the basic documentation for how the server-side stuff is structured then adding new races isn't too difficult. Since the Object Restructure a lot of the server-side stuff is a lot less hassle. The client-side stuff is probably the bulk of the work.
In any case once a "complete" version of the emulator is released it's very likely you'll see sites like this one with sections for server side mods to add features like this with ease. In short it's very likely a lot of big server communities will have this feature once the emulator is released.
Specifically which new races are chosen will depend on how deep their customization can be. (You'd want them to have roughly the same amount of options as current races which not all of the non playable races have. For those that don't there would be a lot of new art assets needed to get them to the same standard as the original races.)
With that said there are a few races in the files which may have been intended to be playable like Kel Dor. Unfortunately they only had 3 mask models to choose from, so we'd need more to make them worthy of being playable. This goes for quite a few races. It'd be nice to unlock some of the unused assets too like Sullustan hair and Wookiee beards.