@Tony Vargas That would certainly work. But I think I'm starting to lean towards not using reactions for this Fighting Style. Each fighting style seems to focus on providing a passive bonus in some way. Dueling give +2 to damage, GWF gives rerolls on 1 or 2, Archery is a flat +2 to attack... none of them seem to require any additional effort from the player outside making attacks.
Likewise the Defense Fighting Style gives a flat +1 to AC. That one doesn't scale with level, but I'm thinking it probably doesn't need to. If bounded accuracy is maintained, it is always relevant and it actually does scale, though it scales with how many attacks come your way, rather than the number you dish out.
So I think an ideal Protection Fighting Style would also be passive in the vein of the Defense Fighting Style. It should scale based on attacks directed at your ally or allies, rather than the attacks you make, and not require any special attention from the fighter. At least not anything more than positioning and fighting, which is the whole point of fighters.
Personally, I really like the idea of providing cover to allies within 5ft of you. It stacks with normal AC, but not with itself (the better cover is taken, rather than combining into superior cover). It also has the benefit of lacking undue complexity (which is in line with the preference towards streamlining and natural language in 5e design philosophy).
But I also like the idea of preventing advantage on attacks against allies within 5ft similar to the Wildhunt shifter ability. That would help in situations such as surprise or against invisible attackers, and prevent things like sneak attack against allies. But I don't think that in itself is robust enough to justify a whole fighting style. On the other end of the spectrum, forcing full disadvantage on attacks against allies within 5ft seems too powerful for a fighting style (even if limited to only one ally within 5ft), especially if we maintain that a Fighting Style ability should be passive and not require action economy expenditure aside from what a fighter is already going to do.
So that's where my thought process is right now:
-Protection should have a passive bonus to an action a fighter would naturally take during combat, even if they did not have the fighting style
-Should not require action economy to activate aside what a fighter will already be doing (thus should not require a reaction or other type of action to activate)
-Should likely scale based on number of attacks directed at a target, rather than number of attacks the fighter can make