Minor Factions: a focal point for trade, diplomacy and intrigue

What really kills the minor factions in ES2 is that they quickly become irrelevant to the game. Usually a few dozen turns into my games all of the minor factions within a major faction's starting constilation will be conquered or assimilated, effectively removing them from the game. I don't think there should be a peaceful option to aquire the minor faction's system, and players will need to fight over who is suzerin for their assimilation bonus (kinda like in civ 6 with city states).

that change along would really change up how they play in game and help keep them in play throughout the game (pending galactic tyrant/craver swarm). 

Besides the other points you listed I would like to see different governements have different ways to get the assimilation bonus of the minor factions they're suzerin of. For example 

• Federations, Republics, and Democracies allow a number of representatives on their senate from the minor faction in exchange for said bonus (while they're suzerin)
  • Federations allow up to 5 representatives, Republic and Democracies will allow 2:1 based on the minor factions population.
  • Federations and Republics could also affect other player's standing with the minor faction with influence and gold respectfully.
  • Assimilated factions will count against your max colonies. Democracies ignore this rule.
• Dictatorships will have to conquer minor factions for their assimilation bonus, but do it at a lower cost and can still trade and do other diplomacy.