As for a warning that a FOB is under attack. I would have it through the command system. Like have it state "FOB Geronimo" is attack. And the the team that already has is control would then have to call off their attacks at other FOBs to defend it, or sacrifice it for the capture of another FOB.
Also EMT, is there going to be any indirect fire support for the squads? Or is it going to be purely shoot and scoot with the infantry. If no on the support that would play into an even bigger infantry based understanding and tactics... Like have and designating base of fire elements, machine gun teams, rocket teams and so forth.
I like the map size and the spawn locations but as for how far the FOBs are from each other. They really wouldn't be close enough to really support each other in the case a FOB gets attacked.... Furthermore are spawns going to be unlimited or maxed out like say 250 spawns?