Well Unioners definitely need a resurrection, but I would say that instead of focusing on a war with Hessians, you should try and focus on Bundschuh instead, thus giving more opportunities to two smaller factions. The Hessians have enough gameplay enemies in their ZoI to be occupied and active, Bundschuh have it harder and could use more opportunities to pew. But also, the more important argument is that the Unioners double deal on the Corsairs and the Hessians by dealing in both Artifacts and Blood Diamonds to get money, and I think that's a good part of the Unioner RP that people often forget about. It's why open hostility against Hessians doesn't really make sense for them in RP, they lose out on the whole deal rather than gain by suddenly having a powerful enemy and also losing a stable avenue of income. The only reason I can think of that would make them consider this would be a full alliance with Corsairs, but I hate this kind of developments because they basically mean the smaller faction is just an extension of the bigger and is defined by their ally rather than by their own RP and diplomacy. It kills the differences between unlawfuls too much. It's exactly why I was glad to have Hessian-Outcast alliance removed in the past.
So what I'd suggest is that Unioners play a part of the opportunistic, pragmatic pirates who have no problems cooperating with pretty much anyone who will let them make a buck, while also paving the way for Corsairs in through the back door into Munich and Frankfurt as a result of their dealings. This naturally will annoy the Bundschuh who don't really want Corsairs in Rheinland due to idealistic reasons, so they will try to sabotage Corsair operations there which will make the Unioners angry, and everyone can take that line of RP from there. Hessian part in all this would be to try to keep good relations with Bundschuh while either not being aware of Unioner back channel deals, or pretending not to know because the Hessians would also have good interest in the Blood Diamond route into Liberty to stay secure. They would remain firmly neutral with Unioners though, wouldn't be surprising if Unioner went past Corsair and Hessian fighting and pretended not to see or hear anything, while Hessian seeing Bundschuh and Unioner fighting would do something like "run to get help" (e.g. go away to somewhere else to avoid annoying either the Bundschuh and the Unioners).
I totally agree Unioners should raid Texas some more though. A couple of years back I was in one attempt to remake the Unioner faction, we raided Texas a lot during RM invasions, picking off all kinds of lone ships who were trying to get to the battle, it was good stuff. The war killed any traders going through Bering, so we smuggled blood diamonds and artifacts to Rogues instead, sometimes in same convoy. Fun times.
Unioner vs Xeno feud is also incredibly under played, XA would probably be happy to have a turf war now that they hunt Junkers in Texas, etc. so the Unioners have two opportunities for interaction there.
