Another cool thing to potentially add are "Parasites" similar to the Go'Auld in Stargate or any number of other sci-fi races.
To do this you'd have a special civic which would cause another (chosen?) race spawn on your world instead of your own. These "hosts" have their own traits separate from your own (which enhance the hosts).
What would be the drawback?
It sounds like you want to have your cake (the others traits) and eat it too (adding the Parasites Traits).
Basically you would be getting a broken amount of Trait points: Both races of this unwilling partnership would get the 2 Free Trait points as well each benefiting from the Trait point additions. And you could do very broken stuff, like leader Focussed Symbiote with Production focussed host.
actually this is already a thing in the update kinda were you have a second subservient species on your home-planet that is strong and dumb. look at some of the twitter update threads.
Totally different. Those are like if the neanderthals survived.
This is "you are a parasite" - you can take over other species bodies and use them as hosts.
That is what you are saying. My headcanon could differ easily. And all we got thus far is a single description on a non-finished part of the game.
Consider:
Goa'uld do not actually inhabit each and every Jaffa and Human under thier Influences. There is only very few Goa-Uld, having conquered at least 2 species. While Jaffa are very solidly enslaved (the gestation cycle of a Goa'uld lava is a biological requirement) this could be mimiced by Synchro Evolution as well.
With SynchEvo you could easily make the main species non-adaptive leader focussed. And just colonise with the lesser species (we will have to see if the traits will actually allow that).
If the parasites do inhabit every last member on the original planet, what you are actually playing is the Parasite Infested Host Species. About the only mechanic I could think about:
After a conquest non-infested pops aquire the "infested" Trait. Upon getting it that trait will reroll thier Ethos once to conform to the Government Ethos. Basically you have a much improoved conversion after conquest. But that is it. They still diverge normally after that and if you loose em and they get converted, you have to correct that the hard way.