Ok so Barb cities are easily taken on the world map without walls. But it needs to be taken into account that many barb setttlements did not have walls. A more realistic option would be to giv ebarb settlements more soldiers in setttlements, but weaker defences. Also certain Barb settlements could start off larger from the beginning of the game.
Possibility of making barbs into 'mini nations?' These nations would fit into their own diplomatic group. THere would be many mini- nations ideally. As you say barbs as one diplomatic entity would be unrealistic. With the adition of more civs in BTS maybe this issue will resolve itself, but there will be extra barb cities anyway and the increasing need for diplomatic organisation.
In response to the Issue of celts in Ireland Walter i'd like to say that their strongest settlements were in central france, andd so celt civ should be there, next to franks. As I said the Franks and celts would be surrounded bt barbs, and therefore would have to unite france/ Germany in conquest/ possibly in the future diplomacy. The situation within 'france' was hostile between tribes. You might even consider removing the Franks as a civ and making them a powerful barb city, leaving Brennus to unite 'France area', whilst being subject to pressure from Italy's troops. Franks could be an emergent civ if you add revolutions mod.
In response to your idea on weakening Mongol troops in desert Fachy, this would be unbalanced.
Walter RTR is a realistic mod, check it out if you haven't already
It gives excelent info on the roman army before and after marian period.
Antazioch marriage to families at other end of world wouldn't likely be possible . A clever AI would realise that maintainance costs would be high. The AI could be convinced that you were 'giving' it the cities. But AI won't accept cities which are too far away if you've ever tried to give cities like that. In other words it would be like an agreement where you giv ecities to the AI, the only difference would be that the AI would then 'die' and you would become the civs new master.
Slave revolts- slaves should be about strength 3 methinks, but after copper is developed could get bonuses against melee units. They are barbs and should get the normal barb penalty, but also their lack of training should mean they are easily beaten by trained soldiers. A large stack of warrior units that is trained shouldn't have much trouble beating them.
Once iron is developed some of them could be given swords. Also when horses are developed they should have some kind of weak horsemen unit.
Slaves could be altered so that the more slaves you have in your empire and the more you force labour in cities the more slaves you have to face in battle.
If you don't force labour or have any slaves the chance of revolt should be virually zero.
1 slave should be equivalent to 1 unhapiness in cities resulting from cruel opression.
Each slave unit/ unit of unhapiness should add a 0.15% chance of revolt in your empire. Slave revolted regularly in the American empire because America used more slaves than any other civ has in history (even more than the Romans). THeir economy depended on slaves running the plantations in the South.
Once the unhapiness from opression goes away so should the chance of revolt
Slaves should have half the maintaince cost of normal units because thy are not paid and in most cases are poorly fed or not at all. THere should be opertunities for slaves to be better cared for later on, and as a result their productivity and maintainance costs would rise. Also slaves should be low maint. for balancing issues. Slavery is supposed to benefit your economy, not crush its growth. The chance of revolution every turn should give some benits over just employing conventional workers.
Serfdom- Slave revolts could be calculated by considering two factors: civ population, city unhapiness and overall food productivity.
The percentage chance to win bar should also say how likely you are to capture the slaves once the battle is over.
Nukes- H bombs/ Abombs. Possibilty of developing H bombs after a fairly late tech. For each type of bomb a 'small' and 'large' bomb could be available. A large H bomb would be the best, but a small H bomb wouln't be as good as a large A bomb. In Japan a big bomb was used in Hirishima and 'little boy' was used in Nagasaki.
Stealth bombers- bunker buster bombs could be used to wipe out strategic targets, eg Iran's nuclear facilities lol. 'Cruise missiles' could have a smiliar effect of destroying particular buildings.
Chem weopons could start epidemics in a civ, but certain tech would reduce the effects of chem weopons. Eg their could be 'inoculations' tech and also bio warfare tech to reduce effects/ duration of epidemic.
Anaztazioch- I like the idea that Mercantiliam should reduce pandemic chances. Also the more merchans you have the higher the chance of pandemic methinks. In other words any foreign trade should increase pandemic chances. But a civ should eventually gain immunity from certain diseases, eg measels, cold, AIDS (lol sick but true. Look at African women they are less likely to get AIDS than western women because they have been exposed to AIDS in certain parts of Africa).
Like you say Walter MTW 2 cant be rellied upon for facts, but it gives useful info on where barb settlements should be on the world map and uses a realistic diplomacy system. Barb settlements in MTW and MTW2 do not all have walls early on.
Roman troops- Although not hist acurate yet i'm sure you'll have better luck when you rewrtite the tech tree.
Note that Andreas is meeting great great problem with your Hermitage class building. As Turkish architecture is very curved, buildings are very uneasy to create. 
This was not fun at all. THe navy I build in retailiation was only strong enough to reppel the attacks by sheer fluke, and more ships would have ended my tiny fleet 
