After looking at some suggestions and civ perfomance as well making all more complete:
General
Elite Steppe Lancer upgrade cost decreased to 700 food, 300 gold (No much explaining that is badly expensive).
Heavy Cavalry Archer Upgrade food cost decreased to 800 food (Same reason as Steppe Lancer)
Cavalry Archer training time reduced to 25 seconds (Many people see them as useless due to their training time).
Britons
Yeomen grants extra attack to Town Centers and Castles (Too incomplete tech in that regard, and helps Britons defensively).
Byzantines:
For the love of God just give them free Town Patrol cmon lol
Bulgarians:
New Bonus: Siege Technologies cost 50% less
(Elite) Konnik attack rate reduced to 2.2
Bagains grants 3 melee armor to Dismounted Konnik, food cost reduced 700.
Free stone mining upgrades
(This civ just need love, a lot of love to be honest).
Burmese:
Arambai gold cost increased by 5, Reload time increased to 2.4, accuracy reduced to 25% (They aren’t that broken as people wants them to look, but just prevents them to be oppresive).
Celts:
Stronghold includes Town Centers (Too incomplete tech and helps the defensively).
Chinese
Rocketry cost increased to 900 food, 800 gold, but also grants the extra attack to all archers.
Chu Ko Nu cost raised to 50 wood, 40 gold.
(Chinese is the classic mainstream civ and can be nerfed a bit, but you will ask hey why the extra attack to foot archers, well is because I feel like Chu Ko Nu are kinda too good in mass (and people screaming with Arambais and Leitis when this guy exists)and lol I can’t remember a game where someone used other things outside of them, and extra attack to foot archers is unused yet, yes i a great buff to their arbalest but it comes pretty late, requieres a castle and a good amount of resources, see this as a counter buff and to descentralize the Chu Ko Nu use).
Cumans
(Elite) Kipchak attack raised to 6, Training time increased to 25 seconds but then Steppe Husbandry affects them and Genitours.
(Kipchak is a good unit but feels lackluster).
Franks
Foragers workrate reduced to 20%
(A small nerf won’t hurts them, still a good civ for all)
Indians:
Sultans allows Elephant Archer at the Archery Range (Still a unit that needs something)
Italians
Dock tech discount reduced to 40%
Genoese Crossbowman training time reduced to 16 seconds.
(This civ feels better now but still needs something, a nerf in water where the are kinda OP and a TT reduction to their UU which is anti cavalry helps them, especially when they lack halberdier).
Khmer
Farmer workrate reduced to 5%.
Battle Elephant speed bonus reduced to 10%.
Tusks Swords wood cost raised to 400.
Ballista Elephant (Standard and Elite) attack increased to 10 and 12, and also affected by the BE speed bonus as counterbuff.
Double Crossbow affects Galleys and (optional) foot archers.
(Dear Khmer, the most controversial civ ever, just unreasonably strong in team games but fine in 1v1, currently there’s a bug with Hand cart that affcets villager’s speed and that probably made Khmer too OP, but once that gets fixed, their farmers will be fine still a further reduction of the farmer workrate is fine, BE speed bonus and Tusk Swords yes need a good nerf because their BEs are unreasonably strong and crazy fast, but Ballista Elephants need a buff yes or yes, and in regard of the Double Crossbow effect, what weapon use the Galley and Arbalest? and it can compensate for the lack of Thumb Ring and Shipwhright).
Koreans:
New Bonus: A small wood trickle benefits them in the game (Like 0,3 wood per 5 seconds)
Shinkichon grants +2 range instead of +1
(Korea needs more to be honest, is a good civ if properly used, but they feels so UP)
Portuguese:
Some kind of eco bonus lol (like starting with +150 wood or better, 100 wood and 50 stone per age)
Arquebus allows Organ Gun at the Siege Workshop.
(The worst civ here LMAO, needs a buff yes or yes).
Slavs:
Boyar (Standard and Elite) +1 melee armor, speed increased to 1,35 and cost adjusted to 65 food, 70 gold)
(Boyar just outclassed by Teutonic Paladin so this gonna helps them to be more on par with Paladins and should defeats the Teutonic and Frankish one in 1v1)
Turks
New Bonus: Blacksmith Technologies cost 50% less food.
Elite upgrade of the Jannisary and Artillery tech affected by the Gunpowder tech discount (With cost adjustments to prevent some unbalance).
(Outside of Arena these guys are underperforming, and it buffs the fast imperial that I like a lot).
Tatars
Economic Upgrades cost 50% less food
Silk Armor provides +1 melee armor
(Not much doubts that they are lackluster in early game).
Those are my ideas, any more can come in the future, thoughts?