While I'm not against an alliance with the fallen, story wise it's not feasible, the atrocities the fallen have committed against civilians, I'm sure there are many who hold a grudge, and I'm sure the feeling is mutual since guardians and the awoken are responsible for the deaths of countless enemies
Plus we can already get our hands on fallen wepons, queen breaker is a wire rifle, dregs promise is a shock pistols and Lord of wolves is a shrapnel gun
What if there is a threat that affects both the City and the Fallen and the leaders where forced to work togther to survive extiction.
Similar to what happend between Humanity and many Covenant troops.
Some how I doubt the fallen would still be open to an alliance with us, the moment they got into our solar system they have been he'll bent on our eradication, it seems they would rather die than ally themselves with humans