Duke William of Normandy
King of England & Unofficial Welcoming Committee
That could actually be pretty interesting. When you conquer all the Cities of a Foreign Civilization, gain the ability to buy/produce their Unique Unit and Infrastructure.The only interesting thing I can think of that they could do with Rome (in Civ7) is cannibalize other civs' abilities, since that's pretty much what they did (and did well) in real life.
Make it so that it's not too overpowered(you have to take all the Cities instead of just the Capital), while still powerful.