Ok, quick. Long list of assorted ideas off the top of my head:
America
Film Studio: Tourism applies to civs in all eras; modern era is a silly requirement
Arabia:
Mamluk: Requires 5 iron instead of 10
Australia: Reduce production bonus from being attacked to 75% instead of 100%
Aztecs: Eagle Warrior, reduce to 26 strength from 28. Tlachtli gives +2 amenities base; +2 faith for each adjacency luxury resource.
Babylon: -60% science instead of 50%
Brazil: Gets 15% of the GPP required to recruit the next GP.
Byzantines: Reduce bonus of cav to 75% damage vs cities when applied. +2 strength per holy city owned
Canada: +1 favor/ for each 50 culture instead of 100 tourism (absolutely useless). +1 extra favor per friendship and alliance. Mountie available at Natural History.
Cree: UU reduced to 35 production instead of 40
Dutch: River bonus applies to all applicable districts
Egypt: Cities on River gain a free granary, and +1 gold to cover the maintenance.
England: Start with boost to Industrialization. Gain +3 iron every turn when iron working is researched.
Victoria: Gain an inspiration when building a Royal Navy Dockyard for the first time on a continent
France: All wonders generate +2 culture. Chateau +2 culture base
Gaul: Combat bonus does not apply to attacking cities.
Gran Columbia: Movement bonus only applies to Military units until Engineering, when it applies to all units.
Georgia: Cities that follow Georgia's Religion gain +2 faith per level of wall. Gain an additional military slot when in a Golden Age. Can declare Protectorate War at Political Philosophy.
Greece: Increase cost of Acropolis to 75% of a regular district.
India: Stepwells give +2 housing. Your majority religion gives +2 amenities. Other religions also give +2 amenities if city is 15 pop or larger.
Khmer: +30% production to aqueducts, dams, and water mills. Cities that follow your majority religion have +10% to growth. Domrey 48 strength.
Korea: Increase cost of Seowon to 75% of a regular district.
Mali: -20% to buildings, instead of 33%. Malus towards units stays the same.
Mapuche: Can build Chemamull on Charming tiles too.
Mayans: +1 Amenity, +1 housing to cities with a government building
Nubia: Reduce bonus towards ranged unit production and experience to 25%.
Russia: Increase cost of Lavra to 75% of a regular district. Reduce Great Writer points to 0.5, and increase artist points to 2.
Scotland: Happiness bonus affects all yields, instead of just science and production.
Spain: Start with Foreign Trade boosted. Mission available at Theology. Gain 2 additional trade routes @ Exploration.
Sumeria: War cart reduced to 28 strength. Can form alliances at Defensive Tactics.
Sweden: Also gain 100 gold x era for each GP recruited. Increase slots for its unique building to 6 slots.
Globally (will never happen I know):
Reduce scaling of chopping and pillaging by 75%
Reduce Yields of Occupied Cities by 100% for 10 turns (back to 50% after)
Make catas and bombards stronger!
Some thoughts:
America: Pretty sure how the film studio works is that it applies double tourism to any Civ that has reached the modern era
or later. It's a misconception from poor wording that it only applies for the modern era. How I'd rebalance Rough Rider Teddy is actually through the Rough Rider unit, which I think has a really high opportunity cost for its era. Make it a Cuirassier replacement so you can upgrade into it. Bull Moose Teddy is obviously fine.
Arabia: Honestly, I think this Civ is fine; in fact, I think Arabia should be the benchmark for how strong Civs should be. That said, I like your idea for the Mamluk and makes the unit better without breaking Arabia. One other thing I'd love to see for Arabia, even though it's not a balance change really, is an alternate leader with Harun al-Rashid that has a trade/culture focus.
Australia: Honestly, nerf the LA to 50%. 75% is just a slap on the wrist.
Aztec: Agree with this, but I think you'd also have to reduce the production cost on the Eagle Warrior or that unit would be pretty rough. The idea for the Tlachtli is brilliant.
Babylon: Fair, but I think the real problem with Babylon is just unlocking techs super out of order. The way I'd change them is make it so if you complete a eureka, you don't automatically unlock the tech; you have to complete the prerequisite techs as well, at which point the tech auto-completes if you have the eureka. If that's too much of a nerf, you could even bring the default science penalty down to 40-30%.
Brazil: Honestly, maybe increase tourism to the city you're running Carnival in while the project is active after researching a late-game tech/civic? Say, Flight, Natural History, Mass Media, or Professional Sports? Not a crazy modifier, maybe like 15%. I feel like buffing Amazon or Magnanimous too much would make the Civ broken. The main problem Brazil has it that the Street Carnival (and Entertainment Complexes as a whole) are just kinda... awkward to build. Making them strong in the late-game culture would give them a pretty valuable niche that pushes you directly towards a victory condition, without overruling their "jack of all trades" design.
Byzantium: Fair.
Canada: Like this, but I think the Mountie is fine as is.
Cree: Cree are a pretty balanced Civ, but the UU could certainly use some help.
Dutch: Grote Rivieren is most certainly not the problem with this Civ, and it's a pretty great ability as is. In fact, the Dutch would at least be an A-tier Civ, if not for Radio Oranje, which is near-useless. Either buff that into relevancy (increase the culture and make the loyalty bonus not a complete joke) or replace it with an actual ability.
Egypt: That sounds insanely powerful. I think just buffing Iteru slightly would do the trick, if not for wonders (for fear of power creeping France) than at least fairly significantly for districts.
England: The real buff England needs is a naval unique unit that isn't a complete joke. Every day I pray we get the Ship of the Line back (or at least the Longbowman! I'd take it!), but if that's not gonna happen, then at least give the Sea Dog a combat boost. I also think that cross-continent or colonial-based abilities don't really work in Civ, so the Royal Navy Dockyard buff, while nice, is super-niche.
Victoria: Much like with England, the colonial abilties in Civ don't really do it for me, so I think Pax Brittanica sort of has a fundamental flaw built into it. Also like with England, the real buff Vicky needs is a UU buff; if there was a Rifleman unit in the game that the Redcoat could replace, and therefore be upgraded into, Vicky would be so much better.
France: Love these ideas, but I also wish that the Garde Imperiale, much like the Redcoat, had a Rifleman to replace and upgrade into.
Gaul: Honestly haven't experimented too much with domination Gaul so I can't comment if they're too OP, but if it's too strong as is, then I can get behind this.
Gran Colombia: I actually don't really wanna nerf GC's movement bonus, since it's their defining trait. What I'd actually change is prevent Commandante Generals from stacking with normal Great Generals. We patched out that General stacking a long time ago, and it being a part of GC's kit, who don't need it as is, is just silly.
Georgia: I like all of these ideas.
Greece: Honestly, I think they're fine. Much like Arabia, I think they're the exact point of strength where Civs should be. Though I wouldn't mind a Hoplite buff. That unit's pretty trash.
India: I like the Stepwell buff, and I don't think the amenities change is really necessary. One thing I'd like to see is a new feature to Gandhi's LA that could push him towards a diplomatic victory too. Maybe receive a little bit of diplo favor for every Civ not at war as well?
Khmer: I like this idea. Maybe add the growth as part of the Prasat? I also wish we had a Trebuchet unit that the Domrey could replace, and be upgraded into from catapults.
Korea: Normally not a fan of increasing the cost of UDs, but in this case, it might be justified.
Mali: Seems fair.
Mapuche: I like this change, but what really needs to change about the Mapuche is allow units to upgrade into Malon Raiders, and a complete rework of Swift Hawk. That's the only ability in the game that actively hurts the player.
Maya: I like this, but I prefer the idea that lets the Maya chop resources from early on in the game. Maybe also protect improvements from disaster-related pillaging if next to an Observatory.
Nubia: Fair enough, but I think the Pitati Archers need a slap on the wrist themselves too. Way too strong.
Russia: All I'd really change is that you don't get the extra prophet point from the Lavra. It's already way too good for culture, it shouldn't also be a guaranteed religion for them too. Maybe make it so the Lavra also only grants the cultural great people points until you build the requisite cultural buildings in the city too?
Scotland: I think Scottish Enlightenment is fine as is, and your suggestion makes Scotland way too broken. But at the same time, I don't really have any other ideas for buffing them. Rework Bannockburn?
Spain: I like all these ideas, but the real problem is that Treasure Fleets is way too undertuned as is, and as I mentioned with Victoria, I don't think colonization-based abilities work in Civ.
Sumeria: I agree.
Sweden: I like the idea, but maybe make the ability 100 gold + (50xEra) instead. Queen's Bibliotheque doesn't need a buff, her tourism is already insane as is.
Globally: I agree with a flat chopping nerf, but if you don't scale down a lot of production costs in the game universally, especially for wonders, then the game is going to feel really, really bad to play.
EDIT: Also, for some other Civs that I'd like to see changed:
China: Buff Dynastic Cycles back up to 60%, and buff the Crouching Tiger somehow. Ideally, I'd like to see the Crouching Tiger replaced (can you imagine Civ VI China with the Chu-Ko-Nu??), but if not, then at least let them attack from two tiles at half-power (or their melee power).
Kongo: Give them back their 100% bonus to Great Merchants! I get the cultural GP points were too powerful, but a better bonus to Great Merchants would just help their overall game so much (and fit their history pretty well).
Norway: Buff the Stave Church like we've said in previous posts in this thread, make the Longboat more powerful, and (maybe) change the Berserker's bonus (say, they only gain 7-8 combat strength when attacking, but only lose 3 when defending). Also another buff to Berserker's I'll get to in a bit.
Phoenicia: Give the Bireme +1 movement if built in a city with a Cothon. Make trade routes from cities with a Cothon also give +2 gold (internal or external) once you research Mathematics.
Poland: This is a hard one, since Poland's kit is so... weird. Maybe give them a combat bonus against Civs that haven't founded a religion? Make Encampment buildings generate either Culture or Faith? I'm not really sure what to do with these guys, because they're so weird.
Scythia: Let Saka Horse Archers move after attacking.
General: Add new units to the game: Longswordsmen & Trebuchets (Medieval Era), Riflemen and Siege Cannons (Industrial Era), maybe differentiate between Great War & WWII-Vietnam infantry, and add some Renaissance cavalry units. Adding these to the game would buff the Civs that have hanging UUs that don't upgrade from anything, and allow them to be upgraded into instead of needing to be raw-built. This would in turn buff Garde Impériales, Redcoats, Khevsurs, Samurai, Domrey, Malon Raiders, Bersekers, and Winged Hussars (make them a replacement for Renaissance Cavalry, unlocked in Medieval era still).