I've played almost all of them once each by now (17/20), and first of all I'd like to say that in general, the unique tile improvements are pretty garbage. This includes the French Château, the Egyptian Sphinx, the Sumerian Ziggurat, the Spanish Mission, the Scythian Kurgan (though I guess you can use this to get a faster pantheon, but beyond that it's meh), and to a certain extent the Great Wall of China which currently is only worthwhile for the defensive bonuses rather than the tile yields. They don't seem to synergize very well with the unstacked cities mechanic because you have much less available tiles per city to work now since you have to make room for districts (and maybe wonders). Taking that into account and also discounting resource tiles where you obviously only have one improvement to choose from (unless you harvest it), you'll want pretty much all the remaining tiles for farms, lumber mills and mines. There is almost no situation where I find room for any of the Ziggurats, Missions etc since they're not that good to begin with. The only times I find them worthwhile is if you get some isolated flatland tiles where you can't get any worthwhile adjacency bonuses from a farm, but problem is the unique improvements generally also have specific adjacency bonuses to take advantage of which might be hard to do in that case. However, if you find a natural wonder with tile adjacency bonuses on terrain types where you can't build farms (tundra/desert) then placing them there can be nice, same with desert flatland tiles in a Petra city. But otherwise... not so much. The only exception to this that I've found is India's Stepwell, which actually has decent yields but more importantly provides double housing compared to farms, which is pretty important as it effectively adds growth in a more efficient way than just raw food. But the others could all use some work if you ask me
Other than that, if we go into the civs themselves, then first of all you have to accept that some of them have more generic bonuses whereas others are aimed towards specific maps or victory conditions. However, that's just part of game diversity, and every civ should probably be judged under the circumstances where they were originally intended to be played. So with that being said, here are some things I would like to have changed:
France - Catherine's Flying Squadron
This ability is almost completely useless. I mean fair enough the early spy is fine, but the diplomatic visibility is pretty much meaningless. The entire gossip system is pretty uninteresting as it is, so getting to see more of that isn't particularly helpful. Sure, getting to see the other civs' hidden agendas faster is nice, but that's such a minor boost that I really couldn't care less. Not to mention this is completely irrelevant in multiplayer
Germany - Holy Roman Emperor
The extra military policy is solid, but seriously the +7 combat strength vs city states is super unnecessary. Being encouraged to attack city states will for one make every other civ on the map hate you more than ever, but also city states are so weak to begin with that this boost is not going to make a difference if you've actually decided to attack one. I'd rather have this changed to something else completely
Sumeria - War Carts
Yeah you should probably nerf these a bit, being able to insta-kill your neighbour just by building 3-4 War Carts is a bit silly
Kongo - Religious Convert
While I think this ability sounded really cool in theory, in practice it's pretty dumb. For one it's completely out of your control as for whether someone spreads their religion to you or not, but even if they do it's not particularly helpful. For one, since you can't build Holy Sites, that also means you can never ever build whatever worship building the religion you're getting has, and you can also never get any decent faith generation rolling which renders a lot of beliefs fairly useless since they're faith-dependent. I think you should just give Kongo the ability to build Holy Sites like any other civ, but instead only prevent them from gaining Great Prophets. That way they could at least spread around a religion within their own territory once they've gotten ahold of someone else's, and more importantly they could actually make use of its beliefs properly. Granted, this would turn Kongo into a very powerful civ since their other bonuses are already very strong but that's a different matter (and certainly not the strongest civ in the game regardless)
Russia - Lavra
This district feels incredibly untested to me. In theory it's the most overpowered district in the game, but in practice it's also impossible to make it live up to its own potential. The problem with the Lavra is that it generates waaaaaayyyyyyy too many great writer/artist/musician points. Like absurdly so. If you focus on building Lavras frequently (which really, you should), then you'll get so many great persons which want to create great works, but it's completely impossible to keep up with having enough great work slots for them. As a result you can literally have 5+ writers just sitting around unemployed, even with an amphitheater in every city and building every wonder in the game which gives extra slots. It's ridiculous. You either need to give Lavras the ability to hold great works on their own (at least works of writing and music), give us more buildings that can hold great works, nerf the great person point yields of the Lavra (should probably do that regardless if you ask me), or let writers/artists/musicians do something else other than just create great works (which I would also love to see just for making the game more diverse and interesting if nothing else)
Scythia - People of the Steppe/Killer of Cyrus
I'm not sure what there is to say here other than that at the moment Scythia is by far the most overpowered civilization in the game, and even one of these two abilities would be enough to give them that award, but both of them combined is utterly absurd. Discounting the current unit disband exploit (which should be patched since it's obviously unintended), Scythia is still far too strong in terms of military. I'm not sure exactly what the best course of action is, but I think the People of the Steppe ability should just be changed to something else altogether. The double horse production is just too silly. Killer of Cyrus on its own alongside some other, more reasonable bonus should be okay though
I'm sure I missed something but off the top of my head, something like that. I also think Norway should be buffed somehow as they feel more like a roleplay civ as of right now, but I'm not sure exactly how yet
