I like the vex because the vex are still actually in the process of winning in other timelines where the Gardener and Winnower didn't create "physical" manifestations of themselves, so even after the Winnower and Gardener are defeated/subdued, the vex will still be a massive threat. As far as I can tell, only the Cabal and Vex will be of any true threat to us after the Gardener and Winnower's dick measuring contest has concluded.
It is intriguing to think how Bungie plans to continue the Hive and Fallen after Lightfall, and to a lesser extent the Taken and Scorn. The Taken and Scorn are basically mindless, and while some people think they will just "pop" out of existence once the Winnower is ejected from reality, I disagree because I'm of the opinion that the Darkness and the Light are separate from the Gardener and the Winnower.
Back to the Hive and Fallen though, I could definitely see the Fallen somehow amassing a reasonable power basis after the powers of Light and Dark are released into the universe from their captors (Traveler and Pyramids).
Same for the Hive, but as the hive's fundamental deity (the Pyramids) would have lost and thus their core philosophy proven wrong, it's interesting to think where the Hive will go after that. It's unfortunate that Savathun is basically confirmed to perish in Witch Queen, and Xivu Arath seems too attached to the sword logic to let go of it, but perhaps Oryx and Savathun could be revived with curiosity/cunning, and I could even see them becoming allies of humanity. Or, at least co-existing with us.
At this point the only path for fallen in my eyes is joining humanity. They can no longer compete with the other factions due to how crippled they've become. At this point I say they should join humanity and have their forces be replaced by scorn across the game
As for Hive I think this mini civil war is the best thing they could've done for them. Since savathune is now against the winnower and worm gods there's conflict and inner politics between Hive factions like never before