I also want to see Colonization II. Allthough if they expand the Civilization possibilites for scripting events etc etc. Then Colonization could perhaps just be a scenario in CIV?
Anyways, slaves is not necessary IMO, but if the game can continue long after independence, perhaps even civil wars can be included? So then the slavery can be a major part of this... Dunno, but there should be some negative effects of slavery aswell, allthough not in the early years.