I think one of two things could happen...
A) Cao employs him as a general and either Lu Bu tries to kill Cao, more than likely getting found and killed in the process, or servs a position of medocrity for the rest of his life (since we know he isn't a good commander, although a good warrior)
B) Bu leaves Cao Cao and takes over his own piece of land, and more than likely gets beaten down by somebody else, and meets a similar fate.
++ Wayne ++
"Better to wrong the world than let it wrong me!"
- Cao Cao