Thank for the awesome replies, guys! Such a wealth of information!
My aim is to make sure the extra two classes are:
1) something the oft-used term "Hero" would apply to (this leaves out things like Assassins and Thief, I think)
2) different enough from the original four main Heroes that they would be an interesting alternative
3) not overpowered
The ranger Goblin-King shows is very interesting, but when I think of a ranger, I usually think of something more like an archer/hunter. I haven't played much AHQ or WHQ or any Warhammer at all. Or even any D&D, so maybe my understanding of a Ranger is completely off, but the figure shown looks like an Elf swordsman or something, and looking up his stats, he just sort of seems like a heightened version of the Elf warrior from HQ, so I don't think I'll go with that.
The Halfling Thief is tempting because it's so different from the other four Heroes and could have some unique skills and provide some additional strategy to the game... but I'm not sure a "thief" can be a hero, and I really don't understand what he's doing on the Empire side of things in Warhammer.
That Crossbowman is pretty interesting. But I feel like he's slightly bland. Maybe if I gave him the additional skills that a thief would have but not call him a thief, but an Archer or something? Like he could roll an extra red dice for movement and can also... do something else? I'm not sure what.
For my first extra class I wanted to do something like a white mage/cleric/priest. But I found out there was no "cleric" in Warhammer terminology (none that I could find). Not even just a simple "Priest". The closest thing I could find was a "Warrior Priest"... which I'm not so keen on. Sounds a bit like a Paladin. But I don't want to ignore Warhammer universe precedent, so I made up a character card for him:

I really wish I could just call him a priest, though. And take away his hammer.

Oh well.