My opinion for balance and changes:
-I think that building repair should cost metal/stone/wood (or whatever it cost to build the building). Rebuilding for free is a bit too easy and it doesn't make sense from an economic perspective either- it always costs resources to rebuild anything in the real world. If a temple is damaged by 50% and you want to repair it (and it costs 300 wood to build the temple), it should cost 150 wood to fully repair it (obviously on a sliding scale).
-Some civs have a big hindrance in terms of some of the resources it costs to build their buildings. Iberian Fortresses and barracks costing so much stone is tough to overcome at times.
-I think that melee calvary should be much stronger vs. infantry (that are not pikemen). In the real world, calvary was very strong vs. archer formations. I would like to see a bit more balance in terms of melee calvary doing damage vs. non pikemen infantry units (this may cause some balance issues early on with some melee calvary rushes, but most civs are able to build pikemen or spearmen at the start).
-Archers should be buffed a little bit. Their range is great, but their attack is very weak. I understand you need to have the tradeoff as well- higher range SHOULD equal lower damage (and vice versa, low range = high damage). But archer civs are still very weak.
-Some siege options for some civs are very poor. Elephants for Mauryans die very fast and are very vulnerable. I think that buffing elephants to make them somewhat equal to rams would be good. And if not, then at least increase the attack they have vs. regular units (maybe with some splash damage). Elephants need a buff.
-Overall civilization balance- I would like it if there are some different buffs for some civs. There's a reason why you never see some civs being used in multiplayer games. Nerfing the celtic civs is a good start. Either scrap some of the other civs that nobody plays (Carthage) or buff them IMO.