I don't think there was much of a unified African nation that was a threat to the ERE. I think there was more African in-fighting as opposed to fighting against the Romans, but I might be wrong.

The Sarmies should be destroyed immediately when they're in a horde, so should the Goths. Then you could go for unification

Unified Rome seems like the only true campaign completion to me, but that's just me.

The Huns should just be made to horde when you have a stack defending the pass. I did this on H/M, and the stack annihilated all the ultra-aggressive Hun, destroying them (they just kept sending stacks at me, which usually had family members. The lack of any characters ends their lifespan )