Not sure if this is possible, just an idea I had for Custom Factions that would help simulate the Cultists from Endless Legend in Endless Space 2.

Conversion faction affinity: Can Brainwash minor factions similar to Vodyani but without the Shipbound affinity. Instead of granting Essence yeilds after Brainwashing like with Shipbound however, it replaces the Luxury cost of their special action with an Influence cost once Brainwashed. (Intended to make the special actions more accessible.)

Also, factions with Selective Migration as their special action can be used as the target of Spaceport departures from your own system once they are Brainwashed. However, you can only have one system of your own at a time(your home system), however you are able to change your home system by razing your current system(which grants a special colonizer ship that will designate the system it settles as your new Home System.)

In addition, System Modernization effects are powered up significantly. Say +300% to all values coming from System Modernization improvements, this is designed to simulate how the Cultists could build their city up to a higher level than anyone else. Taking enemy systems turns them into Hunting Grounds, but having Hunting Grounds gives you extra production of all FIDSI on your home system that scales with the number of your active Hunting Grounds in the galaxy. (Doesn't count Hunting Grounds of Pirates or other factions.)

Again, not sure if this is possible to mod in, but it's something I'd like to see. Thanks for reading, have a nice day!