That's definitely some good stuff, Tony.
My thinking thus far is that only hirelings with Skill: Warrior add to damage. That NPC healer just isn't that useful in a fight. Maybe a few other Skills also add to damage to a lesser degree, like tracker.
What's in my head so far:
PC Class -> Hireling Skill
Bard - > ?
Cleric -> Priest/Healer
Fighter -> Warrior
Paladin - > Protector
Ranger -> Tracker
Thief -> Expert
Wizard -> Adept
Healer Skill adds healing to Make Camp, and/or speeds up natural healing (which isn't in the rules). Maybe also can provide combat healing, at a cost. Possibly something like "When you call for healing from a hireling in combat, take -2 forward as you try to get their attention. They heal you X."
Warrior adds damage to Hack and Slash, maybe also some bonus to positioning or something. Some variation on Bend Bars?
Protector at least allows you to redirect damage to them, maybe something more as well? Might even provide a Dodge bonus.
Tracker has Follow Trail in some form. I guess it could even be keyed off of Make Camp, like "When you Make Camp, choose one trail leaving your camp site, the tracker can follow it until a significant change."
Expert can pick locks and disarm traps. This one might have to include a roll. Something like "When an expert picks a lock or disarms a trap, roll. 10+ they do it quick, 7-9 long enough to cause some trouble." Tracking and disarming are hard to fit into this model.
Adept could add damage to Cast a Spell, or maybe have some kind of on-demand buff/damage along the lines of the healing, above.
Promotion and demotion should be pretty easy, assuming I can some up with something for Bard. Especially if some of these abilities scale with level, so having your friend's 5th level character as an NPC is better than some 1st level hireling.