Overall, I think the Civs are well balanced. So any balance changes are really playing at the edges.
I don't know how the Mapuche get fixed. They currently work really well as an AI Civ - I've had some really cool games where the Mapuche have just abused me with their Golden Age bonus. It's very cool. But as a human player Civ, yeah, I don't know what to do with them. Maybe they just stay as they are, and we all accept they sort of suck a little bit.
I still think the Cree should get a free starting scout. Their scout is too expensive, which makes the Cree actually worse at early scouting. A free scout would fix that, because on a "net" basis they'd end up cheaper overall, and it would be a cool and super unique ability. I honestly don't know why this isn't already a thing.
I think England's bonuses could use another look. They don't need a buff or de-buff, but there bonuses do just feel a bit of a mess. Personally, I think they should lose the free ships and free melee unit for Pax B, and instead just get a flat discount building naval units and melee units in Cities on foreign continents. I think they could also just get a flat +% to Industrial Zones and their buildings (rather than just for the buildings), with maybe the trade-off being their Coal Plants pollute more in the Industrial and Modern Era. My only nervousness about suggesting any changes is that England has gone through so many rounds of just "suck" to get them where they are now - i.e. pretty fun - that I'm nervous about FXS playing with them again.
Spain and (before someone mentions them) Georgia are fine. I hope FXS don't touch them. I think they would more benefit from just Religion getting another balance pass. France's UI still sucks, but that would surely only require a very minor tweak to improve.
Khmer could maybe use a buff. I play Khmer a bit, and they are really fun. But I do think they're sort of underpowered. They maybe just need some better bonuses around their Aqueduct, although I don't know what.
I think Korea, China and America all suffer from being a bit boring. Balanced, yes. Fun? Meh. But they are balanced, and maybe the issue is more they just don't appeal to me. I think both Korea and China could benefit from having a second unique unit, just so they could be slightly more active Civs, but that's beyond the scope of a patch. I think America's problem is just that Teddy is quite a quirkly leader. The US could probably use an Alt Leader that's a bit more military / industry focused, so you can actually play the US as more of a Military super power. But again, way beyond the scope of a patch.