Originally posted by "Dr Jimothy 13 hours ago ":

What I suggest here might be impossible, a giant pain or somehow already a thing but:
Would it be possible to do a setting where your faction is a vassal of another faction, so any time they become hostile/allied/neutral to someone you automatically become hostile/allied/neutral to them as well?

The vassal idea sounds really cool. Here is some brain-storming on the idea.

Could probably be flavoured in really cool ways and have varying costs.

F.ex. If you're an Empire vassal you'd be a Protectorate unless colony wealth is above a certain value then you're a vassal. If you've got a landed-implying title you're that (Barony/County/Duchy).

The main benefit and drawback of being a vassal is that you get a quests where you're asked to do something as a tribute. Paying your tribute would give you give you a very large relation bonus, failing would give you a penalty.

A protectorate might be asked to hand over 3 colonists to do labour. A vassal might more often be asked to produce survival meals, weapons (or something else the colony has researched). A noble vassal could be asked to provide slaves, and gold to a certain value, similar to the regular tribute collector, which would also provide a small amount of Honour.

Your liege would also have a chance to counter-attack when or shortly after enemies arrive. The probability would depend on whether the raid was a major or minor one.

Finally, what Dr Jimothy said, with inheriting relations of your parent faction.

The consequence of declaring independency could be done either by buying your way out to retain relations, otherwise there would be a severe relation penalty.