Personally, I'd love to see Mali, but it has very low chances of getting in on the first expansion when compare to, say, Persia or Babylon, which is much more well known and influential, but just to make it interesting, I'll list a few reasons why they SHOULD be in the game if not now then later.
1. They were a power in their day to rival the power's of Europe, and European explorers saw them as that. They were the main power in West Africa for several centuries until their downfall (as all civilizations have them) by Songhai.
2. They were made quite wealthy through the salt-gold trade and Mali was one the centers of learning in the Islamic world, which was the most advanced region at the time.
3. They are from a region quite underrepresented in Civilization (I.E. West Africa, or more specifically, Africa) and have a unique culture that is very different from any other culture.
I REALLY think Civ 5 would benefit by the inclusion of Mali just like Civ IV had, but in an expansion, and I'm sure I'm not the only one.