There may already be a mod for this that I don't know about but I think it would be great to have the ability to have NPC's swear their service to you and become a housecarl. Perhaps if the NPC follows you there will be a dialogue option for them to swear an oath to you and then you give them a ring.

Also, the ability to mark where your house is like in the Spouses Live Anywhere mod so that your housecarl will actually be protecting your house.

Anyone else think this would make a good mod?

Possibly, but then I also want the possibility of putting all my housecarls in one house. I only use Breezehome anyway, only bought the others to get rid of some money and become thane.