They do this already with keywords like Counterpart and Detachment. The way I would do it is less restrictive than those. I would create a 2 new keywords called Affiliate _____: or something like that, and something like Faction ______: (the name could be worked on). Affiliate would basically state that this unit can only be deployed if a commander or operative with Faction _______ is deployed. Faction _______ would just say that this unit belongs to _______ subfaction. For Saws partisans he is the only one in charge that I know of so it isn't a great example of how it would work. For Ewoks, units with the Faction Ewok key word would be characters like Cheif Chirpa, Wicket, and there are several other options. I think there needs to be something more than just entourage to make a sub-faction a sub-faction. There needs to be some kind of either limiting factor or bonus otherwise it is just a themed force and not actually a sub-faction which is exactly what we have now with the current set of units.

Ah I see now, I misinterpreted what you said to mean they would be more mutually exclusive.

I think that depends a lot on how they plan on doing subfaction. My idea for how to do subfaction doesn't have that problem. I would make subfaction units tied to a commander(maybe operative as well), while a list could still have units from the parent faction. For example if you wanted to run Saws partisans you would need to have Saw as one of your commanders. You could still put in any of the units from the rebel faction to fill out your list.