Egypt could get more production (and Gold after sailing?) form farms on the Nile (or flood plains). The egyptian gampelay could focus on controlling the Nile, improving the tiles arround the river, and then having an easier way to generate happiness and production (for wonders), which will be converted intro VP.
Babylon could gain growth bonuses on the capital when a city is conquerewd and for each occupied city, as well as a free GP when the capital reaches every X population (5? 10?). The Babylonian player could begin in the middle of a bunch of city-states (as in RFC for Civ IV) and conquer them to increase the population in their capital, which would provide more sience (and therefore more VPs from their scoring system) and great people to either increase the VPs (Great Scientists) or to fill the Royal Libraries with Great Works (Great Writter - reflecting the code of laws and writting/archive infrastructure of ancient mesopotamia).
India could focus on having large cities and in having multiple religions on their subcontinent. They will want to protect the subcontinent from foreign invaders (Ancient barbarians, Persia, medieval Barbarians, Mongolia, European Colonists...) in order to score more points but their UA will not allow them to colonize all of India soon , so they will have to work with the city states that spawn at southern india arround 300BC (Chola/Tamil...).
China could try to expand as much as possible. Maybe their UA could come from the "dynastic cycle" UA presented before (
https://civilization-v-customisation.fandom.com/wiki/Xia_(Yu)) and they face a civil war (not full collapse) when a new dynasty is founded, so they could not expand as fast as they would want. Very flavourful UA IMHO.
Greece will try to build wonders but could also focus on expanding to collect the wonders form the ancient middle eastern civilizations/egypt (reflecting the macedonain expansion and helenization process) and they want to generate GP (reflecting the greek classicalgolden age; much like Civ IV RFC).
Persia will want to conquer all of the middle east and India. They could have an ability saying that cities do not enter an occupation phase, as if they have a free hidden courthouse (to reflect the "liberator" rethoric used by achaemenid kings). If the courthouse is free, the Satrsaps court could still provide gold and maybe some happiness but replace the courthouse instead (and could be built everywhere). Another part of the ability could say that conquering a city reduces the next Golden Age threshold.
Carthage is extremely well presented on JFD's mod exposed before. They will want to control all the mediterranean sea with their large number of water tiles and unique naval units. This would allow them to have protected trade routes to easily generate VPs.
Rome could have a different UA, one that provides population in nearby cities when a barbarian encampent is destoryed (reflecting the expansion towards western and central europe), which will not penalize the population cost of buillding settlers, and cities connected to the capital could have reduced construction costs for buldings in the capital (this will make the player want to create a road infrastructure ASAP). They can still conquer cities arround the mediterranean civs to have more cities connected to the capital.
Japan will want to expand outside the starting core islands but they will face some instability and this process will be slow. The altered UB will help them mantain and develop a profitable empire overseas.
Korea will want to grow their cities fast and generate GP (especialy great scientists) as fast as possible. They will have to "turtle" on the Korean Peninsula (which they will unify buy conquering other CS in there) since the neighbours - Japan, China and Mongolia - are extremely agressive.
Huns will want to destroy the world. Maybe they could not get cities but get reduced maintenance for units (since they have no cities, they generate little gold, except from pillaging), have Horse Archers reduce the strenght and damage of nearby enenmy cities (Hunnic Terror; helps conquering and razing cities faster), have science and culture boost from conquering and/or pillaging (as they begin with no cities - they will want some boost on civ development). Aditionally the battering ram (ugly unit) could be replaced by a "Nomad Pastoralist" settler replacement which does not found cities but moves very fast, has a chance of spawning an up to date mounted unit on a pasture reosurce tile nearby and nearby units heal twice as fast. The Huns could begin in central Asia and have a choice to either invade the East (China), the South (Persia, India, Babylon) or the West (Rome, Geence, Byzantine). This could reflect the several nomad-pastoralist migrations of central asia during the ancient era (scythia, Xiongnu, etc.). Eventually the hunnic player will want to find a core to settle and play a still agressive but more regular game later on.
Byzantine focus on defending their land, which makes sence since they are at the centre of multiple potentially agressive and strong civilizations. Still, they can also expand a bit and declare some wars to fuel their VP condition. The faith generaiton and the aditional belief of the UA will help on that.
The Norse (well, Denmark?) focus on rading the civilizations of the west. This is more effetive the more faith they generated, for which the UB helps (this is akin to the Norse gamplay feeling of Crusader Kings Series). Perhaps they could also gain faith from pillage/conquest. Eventually the boost on faith may help them generate Great people later in the game (reflecting the developments of post-medieval sweden).
Arabia could focus on expanding their religion, and it will be even better if that city is within the empire (as in RFC for Civ IV). Perhaps their UB is not useful for this. This Mazar (
https://civilization-v-customisation.fandom.com/wiki/The_Fatimids_(al-Mu'izz)) may be a nice addition to the civ, more in-tune with the expand faith gameplay (abassid golden age, house of wisdom and all that jazz). They will expand fast and wide, convert everything, and then start to focus on science, cutlure, gold generation, making them the most advanced civ of the world.
France will focus on having great works (there are plenty of innovative artistic and political moment son french history, from Charlemagne, to Louis XI (?), Louis XIV and Napoleon). They will be the proeminent works of art/artifacts civ. They will want to generate GP in their homeland, and to build plenty of wonders, but they may also want to expand to other continents to collect more artifacts.
Spain will want to colonize America (or any other continent) and to convert everybody to their major religion. It just makes sense... They should have a boost on faith, for which the altered UB helps. In fact, all the civ could be replaced by this:
https://civilization-v-customisation.fandom.com/wiki/Spain_(Philip_II). The Firaxis UA is a bit crappy.
England will want to create an overseas empire. They need some source of control and some aditional source of culture, otherwise they get far behind on the social policy scene. Thius is why they could use the JFD's Victoria (
https://civilization-v-customisation.fandom.com/wiki/Great_Britain_(Victoria)).
Germany will want to conquer all of Europe, but they will have a difficult life earlyer in the game. The Firaxis UA of Germany does not fit RFC I think. The idea of this mod is better:
https://civilization-v-customisation.fandom.com/wiki/Germany_(Wilhelm_II).
Russia will build a strong core and develop their capital while expanding as much as possible intro Siberia. It fits with their UB and UU. Perhaps the russian UA could be changed, maybe to provide a better generation of culture and faith in the capital? This JFD's and Jeanboruta's Peter is very interesting and historically flavourful (
https://civilization-v-customisation.fandom.com/wiki/Russia_(Peter_I)). Somehting related to enemies taking damage on their land (as in RFC for Civ IV) would be very nice to see as well.
Netherlands focus on building colonies on located luxury-resource filled areas; their UU will protect them against other eruropean superpowers trying to control those resources. Everything just seems to fit well with the Firaxis build.
Portugal will want to build feitorias everywhere and settle some cities to be able to contuct an intercontinental trade network early on, but they will not have the manpower to absolutely colonize everything (unlike spain or england). Maybe the portuguese original UA is too powerful. They could have this ability instead (
https://civilization-v-customisation.fandom.com/wiki/Portugal_(João_II)). This would also make the portuguese player recieve bonuses for proactively expolring the map.
Mongolia will want to completely destroy opponent civilizations. They can go for China, India, Korea, Persia and even Arabia and Russia. They must be a threat for everybody, something that completely changes the political scope of the world. The 2UU system of firaxis may be useful for them. Still, I would personally prefer to see a UB as well... Their UA is quite strange and I'm not sure it will fit the gampelay. These UAs may be more in-tune with a devastating Mongolia:
https://civilization-v-customisation.fandom.com/wiki/Yuan_(Kublai_Khan); https://civilization-v-customisation.fandom.com/wiki/Khamug-Mongol_(Jamukha); https://civilization-v-customisation.fandom.com/wiki/Ilkhanate_(Ghazan). Personally I like the first one quite a lot. It will allow for Mongolia to focus on unit production and war while still keeping the pace on the tech three. It also makes city conquest easier! Even the UB of that mod may be useful for reducing the absurd penalties of conquests.
Inca focus on developing the land into a profitable and extensive empire. This is a small drift from the normal "let's defend against spain" concept, but the aztecs already revolve on that concept and, in fact, incan history is much older and much more focused on what happened before Spain came in. I think it makes sense to drift the "normal" role played by the inca and have them build the south american powerhouse early on.
Azetc want to not be conquered by Spain... But they can still feed their VP by destroying all barbarians/natives on central america and western USA. It would be nice to have the Jaguar replace a later era unit so they are available when the Spanish come.
Siam will want to control and grow their cities on indochina. I really like the Khmer mod presented by Sukritact (
https://civilization-v-customisation.fandom.com/wiki/The_Khmer_(Jayavarman_VII)), and having the Khmer instead of Siam will make the gampleay longer and more interesting IMHO. This Khmer mod will also make the civ want to produce wonders and convert the lagre population into specialists, which is an interesting idea and much in-tune with the Khmer concept on RFC for Civ IV.
Turkey focus on, well, creating the Ottoman Empire. The Firaxis uniques are really bad... That's why it makes more sense to have somehting like this Suleiman instead:
https://civilization-v-customisation.fandom.com/wiki/The_Turks_(Mehmed_II). They will begin in the middle of a ridiculous ammount of civilizations; they can only expand by conquest, and so they will gain cities of very different religions, but this is helpfull with the referred UA. To keep the empire together, they will have to create a lot of puppets, for which this mod's ability is also interesting. The Hammam will help their cities grow, which will eventually lead to a good development of science and cutlure.
Austria is all about "eating" the european city states. The only change I would propose is to replace the coffe house by this building, for flavour (
https://civilization-v-customisation.fandom.com/wiki/Austria-Hungary_(Franz_Joseph)), while keeping it provide increased GPP. Austria will be a large culture focus, probably pacific and defensive; if they survive Turkey, Germany and Russia (?) and are able to eat many CS, they are in a good position to win the game.
America and Brazil work very simmilarly. They will both want to repel the european colonists from their home continents while also developing their own philosophy, science and aesthetics, as a real nation conceptualized during the enlightenment should do... Here they are more like the Aztec, Inca and Ethiopia of RFC for Civ IV. This will make very interesting inependence wars! Afterwards, America will be a production powerhouse and Brazil will be more focused on culture and GPeople generartion, but they can be rather simmilar in their original purpose.