I think that the Illuminate should've been primarily high tech enemies. Their enemy types would include the Cultists, Overseers, Harvesters, Leviathans, Stringrays, Watchers, the big mechs, basically all non zombie enemies.

Instead, the Voteless would be its own 4th faction. They would've been a mostly zombie like faction, taking aspects from Resident Evil, the Flood (Halo), Dead Space, etc. This would be the horror/scary faction, with their main color scheme being green (Terminids are yellow, Automatons are red, Illuminate is blue). They would include Voteless, Fleshmobs, the 2 newly introduced Crusher and Wretch, and various other zombies, maybe stuff like zombie Helldivers/Terminids, Cyborgs/Illuminate, screamers, bloaters, and more from zombie media. Their outposts and spawning areas would be piles of corpses from all factions that are assembled into these abominations. Their lore and point of origin might be something either like a scientific experiment (could've been made by any of the other factions), or they were a parasitic alien race, and they'd have hostless enemy types, similar to the flood's little squid guys. 

Let me know what you think or what you'd change/improve

My idea for how they could've incorporated both Cultists and Voteless