Well, I will write some suggestions too but just for fun. Remember, what will I write about some changes to these civs for fun, that’s all. This shouldn’t be considered that serious. Sometimes, I’ll mention my serious suggestions too.
Britons: lose access to plate boarding armor, light cav, and cavalier. instead of warwolf, a new unique tech allows them to create trash knights for 100 food.
Byzantines: A unique monastery with a dome that operates faster, and heals nearby units as u suggested. Instead of two-handed swordsman and champion, they get a new unique unit, varangian guards. upgrade’s cost will be the combination of two. varangian guards will be similar to berserk, they can heal themselves and they get 14 attacks just like them. For a serious suggestion, these guys need new voices in Greek.
Celts: Instead of light cavalry and hussar, they get a new unique unit called hobelar with an ax as a weapon that is fast with extra bonus damage against villagers and monks. their hp will be 90 but their attack will be 8.
Chinese: A new unique unit called fire lancer to replace knight-line. this unit will charge a gunpowder attack in their first shots, then, became a regular lancer with 9 and 11 attack. another unique unit for siege workshop, mortars instead of bombard cannons, and get access to hand cannoneer. These guys invented gunpowder, so, they should benefit from it. However, rocketry now doesn’t affect scorpions and loses access to scorpions completely.
Franks: Loses access to cheaper castle bonus. I am actually serious about this one, Franks is already too strong, they don’t need such a great bonus. But for the compensation, a new bonus will give 1 extra melee armor in castle age and 1 extra pierce armor in imperial age to their light cavs. Axemen’s range will be 5 at default, and bearded axe will give them 2 melee/1 pierce armor. This way, they can get a boost for axemen.
Goths: Anarchy will give Cav Archers 10 percent more speed and 10 percent faster attack. Get access to thumb ring and parthian tactics. Lose access to hand cannoneers and bombard cannons. Get access to siege onagers and plate boarding armor. Perfusion, on the other hand, will allow them to create huskarls at barracks but it will only give a 50 percent increase for barracks production speed. The price for perfusion might be a bit expensive too.
Japanese: Make samurai similar to Ratha, a cav archer unit that can switch battle modes but when they die, they turn into an infantry like Konnik. The ultimate unit.
Mongols: Well, this civ needs a new architecture setting and this is serious. Both Cumans, Huns, and Mongols might benefit from nomadic architecture. For fun, nomads became a civ bonus and a new tech allows their trebuchets to throw petards.
Persians: War Elephant became an editor-only unit, and their new uu is savar. Savar is similar to catas, they wreck infantry but also cavalry, and they get a 1 range attack just like steppe lancers. Archers, especially cav archers, are their weaknesses. Lose access to camels. Gets a central asian architecture (this is serious actually). Instead of mahouts, they get a new tech that increases savars and paladins attack 10 percent faster. Get access to caravanserais.
Saracens: Get access to caravanserais. In the imperial age, they get access to masjids which allows them to create imams. Imams converse units slowly but heal fastly.
Teutons: In the imperial age, they get access to crusader knights from stables just like condottiero. High melee attack slow cavalry unit that cannot be converted. get access to siege ram and instead of ironclad, teutonic order tech allows their monks to be 50 percent cheaper, and they get extra 2 melee/2 pierce armor.
Turks: Janissary became a unique upgrade for hand cannoneers just like houfnice, their hp with the turks’ bonus will be 55 and their base attack will be 20 but their range will be 7. artillery now only increases 1 range, which also affects hand cannoneers’ range this time but get access to siege engineers too. the new unique unit will be akinji with high pierce armor, fast speed and low melee armor. their melee armor will be 0 just like woads but this is a fast cavalry unit. they will solve the archers and siege problem for the turks. get access to caravanserais.
Vikings: Elite berserk upgrade will get the effect of berserkergang and a new unique tech called ragnarok will give a similar effect like keshiks to all infantry units and longboats to present their raiding. berserks’ hp will also become 75 with the bonus.
How about that? These became already problematic and broken. This was fun though. 
Edit: For the Celtic unique unit, I proposed axe raiders but hobelar seems better from the suggestion of tommochocolate. Also, I decided to buff Frank light cavs with a new bonus too instead of that castle bonus. Both axemen and light cavs became better but they lose their castle bonus. Wow, isn’t Franks broken or something people? I added some tweaks to Goths too.