Like, an armory container you can supply. Any weapons and armor you put inside then randomly appear on minutemen (depending how many varaties you put inside) allowing you to gear them up as you head later into the game. The big problem with Minutemen reinforcements is they're geared so poorly compared to every other reinforcement units. Minutemen outiks provide no protection, and the Laser Musket is absolutely the worst weapon in the hands of NPCs.

I also think the Castle should have a unique "shop." A training/recruitment center, where you can send settlers to be trained as Minutemen, creating more adept defenders for settlements, as well as training up any current Minutemen to be better as reinforcements, depending on how much you invest into it.