Honestly the only other Civs that I can realistically see getting a second leader definitely have those possibilities. That is I don't see any DLC Civs getting another leader and not all of the Civs in the base game.
America: Philadelphia for Washington

Arabia: Damascus, Baghdad, Medina etc.
China: Beijing, Nanjing, etc.
England: Winchester could make sense for Alfred the Great.
Egypt: any non Ptolemy leader would have a different capital.
France: Versailles

Germany: Berlin
Japan: Tokyo, however I don't think they need another leader.
Rome: Constantinople (preferably Byzantines as their own Civ though and I think Trajan is fine)
Russia: Moscow for a Soviet leader or pre Peter the Great.
Spain: Valladolid for Isabella