Since the 'Leaders' in Civ have always been just names and graphics to hang attributes on, I don't think it matters what they are called. An immortal 6000+ year 'reign' is automatically fictional and even Mythological, so you might as well call the Japanese leader Yamato or the Norse leader Woden as hang any 'historical' name on them.
Now, what would be interesting, is to enlarge the pool of Great People and their effects, since they are actually the closest thing we have in the game to 'real' personages. yes, they are effectively Immortal after you get them (until you Use Them Up) but at least they don't come knocking on the palace door in 4000 BCE!
Add, say, a Great Statesman (see the Mod Rule With Faith - I'm shamelessly borrowing some of their ideas) and/or a Great Politician (I know, sounds like an oxymoron to me, too) which could add external and internal bonuses/benefits to relations with other civs and your own people (trade possibilities, happiness, loyalty, amenities, etc)
Take the current pack of GPs and add some (lesser) Leader-like Uniques: Buildings, Units, and such. Right now we have a whiff of this - Great Admiral Themistocles giving a Quadrireme, for instance, but why a ship-type that Themistocles never saw in his life? Why not instead he gives you a specialized Galley - a Trireme or Trieres with increased Attack Factor against Naval Units so you can safely go hunting those pesky Barbarian Quads?
Hannibal Barca - instead of one lousy promotion, how about he gives you 2 Numidian Cavalry: Horsemen who can move after attacking and Ignore hill movement restrictions?
Marina Raskova - gets you a Night Witch Bomber: a bomber with the range of a fighter, a biplane graphic, but it can land and fly from any plains, desert, tundra or grassland tile - doesn't need an airfield of any kind (the real 588th Night Bomber Regiment or 46th Guards all-woman regiment's Po-2 aircraft could land in a soccer field almost without touching the goal posts)
Since there have been other discussions about 'Wonder Races', how about making a Great Engineer or Architect or other GP Required to build specific Wonders? You want, for example, to build the Eiffel Tower - you will have to get the Great Engineer Gustav Eiffel or either can't build it or pay a fierce penalty for using 'second-rate' engineers.
Just saying, the Leaders are just a source for Uniques of one kind or the other - I space past the animations of all of them after the first 2 - 3 times they come up, so appearance doesn't matter, names don't matter: like the old joke about Boffo Soap: Doesn't Lather, Doesn't Bubble, Doesn't Clean - It's Just Company in the Tub! Same with Civ VI leaders: just company on the screen . . .