Having minor factions not being able to be assimulated right off the bat, but requiring the culture conversion tech (alongside with the assimulation cost being much higher). With the default option in terms of assimulation is a weaker form of "brain wash" i.e. less resources/no evacute (unless you have the ship bound affinity).

With the axis to intereact with minor factions being growing them with developement grants and the usual (like the mod that unlocks all of those actions,good mod I must say).

Another idea is rather odd, I wonder about manpower being basically population, since already it appears different kinds of manpower is kept track of i.e. the visual composition during ground battles(the races) and when you update to having include tanks, some manpower still retains infantry only compostions, compared to freshly generated manpower.

So from these premises (I honestly not sure of the depth of the underlying system already in the game), I wonder if having the "fate" of said manpower affect your systems i.e. losing ships that have a lot of manpower(naturally and via +manpower moduals), will cost some unhappiness. While having said ships stationed in ally/your own systems, will increase the military political idealogy less than having said ships on the offensive.

Kind of like having military personnel stationed out of country still being able to vote(but in this case only affecting the political opinion).

Sorry for the long post, I just like to be able to tinker with politics/diplomacy more. Cheers.