I've always been partial to the idea of a very grounded rebel faction, based on asymmetric power imbalances and a risk to their existence if discovered. They would operate in cells and strike larger factions in small raids. There are lots of really great examples in newer Star Wars media (think Andor, The Clone Wars) of the fractious, often desparate groups who fight an ongoing, seemingly fruitless struggle against tyranny.
There are a couple of barriers conceptually though.
Firstly, it relies primarily on NFUs and particularly disenfranchised, desparate characters who are low on the power scale. This is actually a pretty uncommon character type on chaos.
Second, if this kind of faction gains enough traction, it naturally evolves into a Rebellion, and then a Republic, and there's already a huge faction on the board that fits the latter. This could be addressed by purposely keeping the faction low to the ground and relatively weak, but idk that people would take to that.
Finally, this kind of faction needs a big baddie. Preferably, a big Imperial baddie who hunts and persecutes the rebels. If this dries up, which in the past it has, there goes your activity. I think that there's a possibility that you can sidestep this by making basically every major board player an enemy (e.g. the Sith are enemies because of course, but also so are the Alliance, because they are a politically bankrupt war machine). I have no idea whether that would work in practice anyway.
Regardless, as mentioned above it's a concept that lends itself to having a small, really dedicated cast of people's mains, and I need to become a wayyy faster poster if that's gonna be me.