If they do a second frontier pass I'd absolutely love if they went back and revisited old civs and seriously redesign some of them to at least be on par with new ones. Debblobing India is possible and should be done, they've already made abundantly clear they don't shy away from post colonial civs, break India apart, keep Gandhi (so that Firaxis can keep on the stupid nuke joke) but make him the leader of post colonial India alone. And give Chandragupta a proper Maurya civ. As you mention they could also add another leader while they are at it and give us Chola or Mughal.
There's still civs I want to see back (Haudenosaunee and Assyria) but think that adding a bunch of old favourite leaders could seriously gain some traction even if all of the favourite civs seem to be done. Ramses, Isabella, Catherine, William of Orange, Julius Cesar, Wu Zetian, Theodora, etc.
make it...sort of Civ VI ultimate, all your favorites are back.