Esker
Hero
I like @Undrave 's solution pretty well, but from a fiction perspective it's hard for me to imagine how the fighter is protecting more than one person at the same time, all without even using a reaction. I think I would leave it mostly alone but have it scale better: Something like,
"When you have a shield equipped and a creature you can see attacks a target other than you that is within 5 feet of you, you can use your reaction to impose disadvantage on that attack roll and any subsequent attacks made by that enemy against that target until the start of your next turn."
"When you have a shield equipped and a creature you can see attacks a target other than you that is within 5 feet of you, you can use your reaction to impose disadvantage on that attack roll and any subsequent attacks made by that enemy against that target until the start of your next turn."