This might seem like an odd request, but here’s the rationale: When they added Arrow Slits, they never considered the effect it would have on garrisoned villagers. Before Arrow Slits, Guard Towers did 11 damage, and Keeps did 12. That means garrisoning 5 villagers in each would give you 2 extra arrows either way. So for example, against a Hussar, a Keep garrisoned with 5 villagers would do 2x12 damage, or 12 extra damage per volley after armor. With Arrow Slits, they now only give a single extra arrow, and only give 9 extra damage per volley after armor.
But if you gave them +1 garrison capacity, Keeps would have a perfect 2 extra arrows, with 6 villagers providing 30 extra damage! This increases their garrisoned damage up to 18!
This would make Keeps a much more valid choice at certain stages of the game.
Thanks!