In a hypothetical third expansion, I'd expect something more in the alt leader department than just a single alt. In the first expansion, we got one alt leader, but in Gathering Storm, we got one that was leader for two different civs. They one-upped the first expansion with that one, and I predict they'd want to one-up the second one here. How they would do that is certainly up for debate, but it's possible we'll simply get two alternate leaders for different civs instead of just one. One for Egypt, say, Hatshepsut, and one for someone else, ideally another civ that people want an alternate leader for. Maybe they'd try something a little more innovative, but I don't know what else you can do other than someone that can rule multiple civs or just having more alts.
I used to think plagues in Civ would just be an annoying thing that you'd have to worry about without any solid way of protecting yourself from it. But someone here (I can't remember who

) suggested that perhaps plagues could be linked to housing. If you're short on housing, you'd increase the risk of disease in that city. I'd suggest that pillaged tiles could increase the risk as well, but you'd probably still be losing housing when improvements are pillaged so I dunno, maybe that would be overkill. Really, all I care about with mechanics like plagues is that there should be some way for a player to mitigate the damage as much as possible, so it's not like their civilization suddenly implodes and there's nothing they can do about it. That's why I was a bit apprehensive about natural disasters being introduced, especially earthquakes. How do you predict where earthquakes will show up? How do you protect yourself against earthquakes?
Ideally, new mechanics would also tie into other existing mechanics, so that everything feels connected and the game doesn't feel bloated with features. Maybe most people don't mind if the game is bloated with features, but connecting mechanics to one another can be used as a way to help improve existing mechanics, like how emergencies in R&F were added to World Congress in GS.