At the moment diplomacy with factions seems kind of slow and unrewarding. After progressing with a faction all you get until you reach 50 diplomacy is knowing where are their bases are. Why not do something like at 25 recruitment costs decrease and you can buy basic equipment from them ie that factions armor and weapons. At 50 you can choose what type of unit you want to recruit from a haven and you get a small amount of things to research from them. At 75 you could recruit soldiers already at higher levels, be able to purchase any of their weapons and armor and get access to all of their research. This way I don’t have to wait several hours into the game until I’m able to figure out how to build a shotgun or attach a piece of metal to a stick for melee units (looking at you Cult of Anu).
In my game I reached 50 diplomacy for NJ and Anu after the same fight (destroying a Nest). So in one fight I went from having no tech, unable to MC, can’t build equipment, etc, to suddenly having all that for 2 factions. Felt a bit 0->100, I agree that maybe having some of the items filter in, instead of “dump everything at 50” would be a more elegant solution.
For example at Diplomacy:
20 - recruitment 10% cheaper, can craft ammo (trade?)
30 - basic research shared
40 - can produce basic weapons, recruitment 20% cheaper
50 - moderate research shared, one class + MC
60 - recruitment 30% cheaper, can re-roll recruit or maybe pick, advanced weapons
70 - advanced research, second class + MC
80 - can benefit from their bases for healing/resting (basically flag us as both factions)
90 - can control faction defenders/survivors on haven defence 
Those are just examples and there is a lot more that could be placed there, like lower or faster production, etc…
I agree. Seems like a decent proposal to me. Why wouldn’t a faction not allied but supportive not sell weapons and ammo. But perhaps restricted to what is already in my inventory.