Fireaxis invited a number of people to their offices to play the game and learn a bit more about it, quill18 has posted a bunch of videos about it. I'm only halfway through the series so far, but it's got a lot of interesting details, the first one is here:
With more on the mix and match leaders stuff. The video is B-roll from Fireaxis rather than his playthrough but it still gives a lot of detail. The game looks gorgeous, and he says it actually looked a lot better in person before it went through several stages of compression. I really like the way the tiles you haven't explored yet move up and down showing you the rough lay of the land before you get there. The mix-and-match system looks like it will offer a lot of customisation and variety, and I also expect there to be a whole bunch of broken combinations in there. The second has a lot more detail on how the cities are going to work:
So the districts are gone, instead each tile of your city can host two improvements (possibly more later?) in an "urban district", and more radically builders/workers are gone and replaced with "rural districts" where you build tile improvements that leave the contents of the tile intact. You can replace rural with urban later, and there are interactions between the tiles which should encourage specialisation. There's also a new separation between "towns" and "cities" so your smaller settlements act as supply satellites to the cities, I think? It wasn't clear. There seems to be a lot of visual design variety between the cities of different civilisations - which will look great, I'm sure, but I worry will muddy the visual clarity of the game and end up making you mouse over stuff all the time to see what it is. The third has information on governments and diplomacy (mostly the latter):
The separate civic tree is still there, technology has some kind of mastery mechanic giving extra bonuses for the same tech, and each civilisation (not leader) gets a few unique civics so as you move on you'll keep unique stuff from the ages before. Diplomacy is based around some kind of "influence" resource, which I presume you garner from buildings in some way? You can spend it to take actions, make pacts, etc. but the thing I think looks really interesting is that you can support one side or another in a conflict without getting directly involved which should make diplomacy much more interesting and involved. Another interesting point from this one is that there is a city limit mechanic (1/3 in the video shown), he said that you can go beyond this number but then you start suffering efficiency penalties? I guess this is replacing mechanics like global happiness and corruption from earlier entries in the franchise as a mean to restrain the number of cities you can spam. I'm assuming towns won't count towards this limit and that wonders/building/civics will provide means to increase the limit and/or reduce the penalty for going over it. It will be an important point of balance to get right I think.
There's two more videos
in the series covering "Combat and Commanders" and "Crisis & Legacy paths" which I've not watched yet. I'm not sure what the latter of those is.