Let's say that we get 4 more expansions.
1) A Steppe DLC, possibly mixed with Inland Trade and an improvement of Iran around the theme of the Silk Road.
2) A Myths/Legends DLC.
3) A Lategame/Peacetime DLC.
4) China. By this point, hopefully the Dev's stated intention to be able to do this without a significant performance drop/spec raise will be fulfilled. I have heard some doomsayers positing that it'll just be like RoI again (optimisation was promised but not delivered on), but it seems naive to think that the Devs are unaware of the backlash and that they'll just do the same- I imagine they'll take special care to ensure good performance on the lower-end computers if they make China.
5) This is not so much serious as it is wishful thinking, but an "Indian Ocean" expansion: East Africa, SE Asia and Indonesia, to complete the Afro-Eurasian trade-route systems which defined the Old World at this point in time.
With China in the game, and all the above, I think the game will be done. The grand interconnected Eurasia of the Middle Ages will all be there.