BOS is an absolute, but for the others that's still up in the air. In Fallout 3, it was obvious that the institute would be showing up again except at a much larger scale. On the other hand, the railroad encounter during the "Replicated Man" quest was such a small occurrence. Skip to late 2015 and the railroad makes a huge appearance in the game, me personally I didn't see that coming. Fallout 4 doesn't talk about any foreign factions making moves like Fallout 3 did. In the next Fallout, I believe the BOS is gonna have an appearance whether it's on a large or small scale, since they've made appearances throughout the entire Fallout series. Including a variation of the Brotherhood in some form is a Fallout tradition at this point. Its possible the Enclave might be included but it's not certain. Either than that I don't know, I think they are gonna add new groups along with the new setting. Hopefully they include minor evil factions that are involved with the story, the gunners played a large role in Fallout 4 but didn't contribute to the story whatsoever. The Nuka World wasn't story based which was lame, so that doesn't really count in my opinion. Although, one faction I can 110% percent guarantee will show up in the upcoming Fallout is the Super Mutants.