Vikings are of course a necessity, as are celts. Leaders should be ivar the boneless and vercingetorix. I think spain should be in, not that i would ever play as it, just so i can beat it up. Dutch, portugal, prussia, all important. (i dont think anyone else said prussia yet) But what I would really like to see are the Maori, tribal aboriginees of new zealand, in 1864 230 maori defeated 1700 british imperial forces, it was massacre. The maori are essentially the vikings of polynesia.
Also, if more leaders are added, Julian the Apostate should get to be in it, he was a Roman Emperor who could be considered the last truly great emperor, if he hadnt died in battle rome might have been a lot more succesful in the end. He also tried to resurrect paganism.
Oh, that reminds me, i dont have civ v yet, but if its got religions, they should add in paganism, im tired of always having buddhism or taoism, and ill die before i take christianity or islam.