While I would be the first to say, this is not really needed in the game, I think it would be a pretty cool addition. Here is my Idea:
level 7 like pnp, or taking a page from augment summon, and remove the prereqs entirely.
Active clickie ability. Multiple selector. Allows you to summon any one hireling who's level matches your own (maybe one level lower or something if balance is an issue here). Summonable anywhere in dungeon, like gold seal. Hireling stays around to do one job (sticks around for about a minute or two), then leaves till you call them again.
Cooldown: I was thinking 5 or ten minutes.
Anyway, let me know what you think.