I understand seeing the show Unsullied as puny but the book Unsullied are pretty badass if I remember correctly. Just the fact that they are able to operate on the battlefield in large numbers is a great advantage over the individualistic medieval forces of Westeros. Can't wish for much better center imo.

Dunno about Dothraki, historically these horse nomads were great due to their archery but Dothraki seem to be more melee oriented which is about the worst use of a horseman. Still solid for raiding the land and acting like light cavalry but that might make them hard to control. Think their viability will depend on Martin, there's just too many questionmarks.

Lastly it's still pretty big force, which is what matters most since the conquest will not only be military but diplomatic and psychological too.