So, I love 6, but there's still a few things that would be cool if they added.
First is the disease mechanic they introduced in the Black Death scenario. I'd actually love to see that expanded upon in a new expansion later down the road, with a few leaders really taking it into their abilities(I can already think of one...well, two, technically...)
Next would be the ability to redraw shared borders after a war. Not just take cities, like we can now, but redraw which civ gets which specific tile(though this might be a bit of a challenge to program, especially AI determining what they'd allow considering how the war had gone.)
As for new Civs and leaders, I can only think of one. I'd really like to see the Byzantine Empire represented again, not only with Justinian, but Theodora as well, but not as separate rulers: as Civ's first co-ruler. I just think that you can't really have one without the other. While I haven't thought of any unique districts or units they could have, or what the Civ ability would be, I have thought of a cool leader ability: Justinian's Plague. It would have to do with the disease mechanic I brought up earlier, likely lowering lethality of the diseases and something to do with gaining culture during/after the disease, but it's based off of the real Plague of Justinian, a strain of Black Death that went through the Byzantine Empire, and that Justinian himself caught and survived. That was a rare case, I believe, and would make sense with the mechanic if it's ever fleshed out.
Also, I can't wait for Gathering Storm to come out for the Switch version of 6. Playing as the Maori looks really fun.