My speculation of an expansion pack for Medieval 2 would probably be one the Rennasaince or the Christian Reformation. This would include lots of Gunpowder troops.
Or they could keep it Medieval and keep to the Invasion Name "Mongol Invasion" or "Muslim Invasion" could work... although the latter would probably be controversial seeing the title name haha.

They coul also go back to the Crusades era and zoom in on that part of the map and make it the focus. Kinda hard to speculate when the first game isnt even out yet.