So, we've got Children of the Atom and Institute (most likely) Synths in Far Harbor, which groups do you think may be the rival foes in the next story driven DLC? Name the 2 if you wish

I really hope to see more aliens.

I think the big 3 (Institute, BoS, Railroad) won't be DLC viable given the main story. Aliens could be, but not sure if Bethesda would dedicate ANOTHER full DLC to them given how Mothership Zeta was received. Gunners aren't like the BoS outcasts from FO3 who were already neutral to you, they're aggressors. The forged were just that quest with the Raiders in the ironworks right? Can't see them going anywhere. Minutemen will always be a contender due to being neutral to all characters/choices, and the Enclave is unlikely to return as just a side DLC if they return at all.

That leaves the Atomic Cats, which could have some crazy, power armor related quests. Like finding a secret stash of them for example. It also gives Bethesda an excuse to add MUCH needed customisation expansions to the power armor options we have. Maybe more things not locked behind Science perks. It wouldn't be the faction I'd like to see most, but I have an easier time seeing these guys as DLC candidates than the others above.

I chose Aliens, the Cabbott family thing is just left way open in my opinion. It felt like there was going to be a future mission regarding Lorenzo and the place he visited could potentially be DLC.

A new totalitarian imperial faction out of Ronto spreading their influence into Maine. Joining them makes Preston and the Minutemen enemies.