I think the fallen will definitely ally with us later. Maybe a small group of cabal, like those loyal to calus.
The vex will only ally if their "great puzzle" or whatever it's called is threatened.
I highly doubt the hive/taken will work with us.
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Vex will never ally because we are against the symmetry. They can't simulate us so they kill us
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Well they would at least have a temporary truce, remember when we needed to save VoG from the taken? They just kinda let us in for that. Yes they tricked us into thinking that hunter from kabrs team was in there but still.