Not sure if this was mentioned, didn't read most of the posts, but, have instead of a normal merc maybe a merc skilled in various skills that it could pick randomly. With this could also be classes for each act. In D2 you basically have a Zon for act1, Pally for act2, Sorc act3, and a barb for act5. WTF! Wheres the necro for act 4? This would cover all the main classes for d2.
In a D3 I would like to be able to choose mercs, based not only on def, off, or combat, ect. but why not Pally, Barb, Necro, Sin, ect. This would give someone not used to a class to see what kind of potential their is. Imagine a bone necro backing you up as you fight, or even a trapsin.
I think it would be interesting to see gear that can be used only for mercs. Would make things a bit interesting. Just some ideas.