As far as diplomacy with an [b]entire species[/b]? None of them. There's a few rogue agents like Mithrax and Calus, but we're not befriending an entire enemy race as a whole anytime soon.

We are already allies with Mithrax’s House Of Light , he actually fights with guardians,even against other Fallen. The rest of them one day is going to have to choose peace or extermination , if (when) we destroy Eramis and hunt down her lieutenants, the Eliksni will have no more leaders that oppose humanity. The Red Legion has lost so much fighting Humanity, they may one day decide to make peace with us , but Cabal are not known for surrender ,but we see Red Legion commanders talking with Zevala and Osiris, Calus left and took his loyalist Troops with him The Hive worship death and feed on the light , they kill each other out of love, they are an ancient enemy and there is no hope of diplomacy with them , although Nokris was trying to tempt us to join Savathuun , that was a hard pass by us. If there is one true “Evil “ race in Destiny it’s the hive The Vex are just beyond our comprehension,they have one goal and every unit is committed to that, they are also beyond diplomacy

House of light

So. Taken, Scorn, Hive, Vex and House Salvation (that is their confirmed name) would not be open to forming an alliance with the forces of the Light. The Vex have used us before to help themselves as shown when the Taken invaded the Vault of Glass but other than that, it’s not gonna happen. House Judgement and House Light would defo be open to alliances with the Forces of the Light. The Awoken are already in an alliance with us. The Cabal are really odd. Technically we have 3 Cabal factions. Calus’ Loyalists who wouldn’t join us in an alliance against the Darkness as shown by Calus’ outright devotion to it. The we have Caiatl’s empire who may form some alliance based on the fact Calus does side with the Darkness and we don’t like the Darkness. Also Calus ain’t exactly an ally per say. Then we have the Red Legion. Now I count them as a third faction based on the line we get from destiny 1. “When the Cabal leave for war they’re kicked out of the empire and their only way back is to return glorious” (I’m paraphrasing). They may have the Empire’s best interests at heart, but they’re no longer with the empire and considering how we’ve managed to kick the snot out of all their leaders, they may be open to another option after seeing the Darkness take away 3 of their 4 main footholds (I don’t count the Tangled Shore here on the basis they don’t have a forward base of operations there). But that last one is merely speculation giving reason to them joining us.

The Fallen are the most likely to accept diplomacy, though mostly in groups/houses rather than a whole species alliance. It is possible with the Cabal, but only Calus's loyalists as far as we know. Due to his affinity for the Darkness though, this is unlikely. His daughter, Caityl (I think that's how it's spelled), might be more willing if the Darkness chipped away at her empire. The Vex are an entirely neutral force; they will fight all other races if and when necessary. If they are to ally themselves with us it is solely because the Darkness is an overwhelming enough threat to stop even them to the point where they require outside help (which was the case based off the events of the Taken King and Vault of Glass). The Hive, Taken, and the Scorn are all contradictions to what humanity stands for. There is no chance of peace with them.

Cabal going by the ViDoc...