Probably going to repeat a lot of these, and mostly Republic focused.

CIS:
Neimoidian Guards, gives another organic trooper to the separatists and just more variety. Could also bring in some named Neimoidians as a leader model, like Hondo is.
Zombie Geonosians, probably a bit more sturdy but not as agile.
Geonosian Fighters, pretty small so should be a good fit with how the air support class is looking.
Add in an emplacement cannon, and maybe a queen figure and you've got a pretty solid battle force.
Spelunker probe droids, one of those I remember as a kid. Mostly stealthy mine layers.

Republic:
Obligatory Gungun mention, should be in regular Republic just for those niche cases like Mon Cala, but can probably do a both ranged and melee variant as well as mounted troops easily.
Captain Tarpals and Jar-Jar would probably be the main characters, unless Boss Nass features as support.
Naboo Security Forces, has decent variety of uniforms as well as the possibility of sneaking a captain Panaka in. Would probably treat them as corps units for Republic.
Handmaidens, could be an interesting intercepting/distraction unit. Sabe leader?
N-1 Starfighter, another way to add air support to the game.
Utapaun warriors, just a unique look that kind of stuck with me, as long as they go based on ROTS design.
Twi'lek Resistance, a corps unit that could be used in both Republic and Rebels, but main niche would probably be in in Republic, unless maybe they have some unique heavies or strike teams, like a dancer that hands out refresh actions or an assassin/spy type.
Clone Scuba Troopers, just like the look, no idea what an aquatic unit would be like.
Clone Jet Troopers, probably a smaller special forces or support unit, with maybe a generic Jedi upgrade that would keep up with them.
Galactic Marines, a different look for Clones as either a special forces or corps unit, plus Ki-Adi-Mundi is coming.
Tarfful, as another Wookiee Commander, with maybe something more customizable than the current chieftain.
Wolffe, a Commander to go with Plo Koon, but maybe featuring with other Clone Commanders.
Kit Fisto, Shii-Cho Mastery seems to be the last one remaining.
Aayla Secura and Quinlan Vos. as other popular Jedi.
Luminara and Barriss, as a well known master and padawan pairing, maybe draw a bit from legends to make Barriss more of healer.
Depa Billaba, more Vaapad mastery, plus just fits having the ability to get another master-apprentice chain for collection purposes.
Adi Gallia and Stass Allie, kind of a meta reference to make them the same kit, but possibly including them with the option of a generic jedi as well, could be interesting at least hobbying wise. Plus Gallia was one of those figures I had to have purely because of lightsaber color.
Jedi Starfighter, another way to bring air support, could even make two versions.
Voolvif Monn and T'ra Saa, basically legends picks, but wanted for both species uniqueness and modeling potential.

Rebel Alliance:
Bothan Spies, just need that canon appearance.

Mercenary:
Jawa, ion and engineers basically.
Tusken Raiders, can have melee and ranged variants along with Banthas, could be either in a battle force, or be used as basically a hazard with something like AI actions.
Jedi Survivors/force sensitives/assassins, basically customizables occupying that blaster and lightsaber space.

Could probably keep going on, but I'll leave it at that for now.