My future-Civilisation fantasies:
Haida, under Cuneah of Kiusta;
Armenia, under Tigranes the Great;
Minoans, under The Prince of the Lilies;
Haudenosaunee, under Jigonhsasee;
Novgorod, under Rurik, Yaroslav the Wise, or Marfa Boretskaya;
Punjab, under Ranjit Singh;
Hausa, under Daurama.
As much as new civilisations, though, I would also like to see new leaders for those already in the game, especially a pre-Romanov Russian leader, a Qing dynasty leader, a pre-Ptolemaic Egyptian leader (as many have said before) and, if the developers want to be especially bold and brash, a semi-mythic Bronze Age Greek leader like Menelaus.