There are far more small fcs than big fcs. Despite the great content we'll be getting in storm blood, what also holds this game together is the community. A Fc alliance feature would make it easier for smaller fcs to connect. It would even be amazing to have fcs more involved with each other via ingame features. Having a fc alliance chat, using each other's housing resources, and heck maybe even having buffs that branch across the allied fcs would be nice.
This is one of the most underrated features mentioned so far Imo.

Basically, each fc leader & selected officers would have to contribute a certain amount of credits to execute cross fc actions. The actions would have to be completely unique compared to the current ones we can already execute. New actions would also be best & it would encourage players to have fc alliances - I'd say up to 3 fcs should be allied.