At the risk of turning this into the "World Showcase Wish List" thread, I'd like to see Greece. You'd have distinctive architecture, for sure, and if Disney needs an IP tie-in for a ride, have the cast of HERCULES take us on a journey through Greek mythology.
Obviously, my dream pavilion would be the old concept from the early 1990s for Russia, which would have instantly been the best pavilion in World Showcase... but we all know the geopolitical climate will not currently allow that.