I would love to see Treach, Ice Cube, Tyrese, Omar Epps or that dude from Paid In Full/Above the Rim (Bird's associate in it).
Treach is a pretty good actor man, Jason's Lyric and Love and A Bullet did him justice.
Tyrese did heaps good in Baby Boy as mentioned several times earlier.
Omar Epps is just an all round great actor, House (The T.V. Show), Love and Basketball, Juice + many more.
Ice cube's list of movie features/stars is expanding by the year man... he's good aswell.
The thing about all these actors (with the exception of Tyrese) is they all knew tupac, and they all pretty much knew him on a personal level i'd say. So they would b able to understand a bit about how he'd want the role to be played.
And after all, Treach was like tupac's closest friend no?