The 13 civs are overall in a very good state. Goths (with their archer-resistant infantry) and Celts (with their siege bonuses) are historically really inaccurate. But they have become so iconic that they can hardly be changed anymore now.
Samurai are a bit bland and mostly useless, so a smaller buff would be good there. Either the nice idea of UU-resistance or they could charge (run) the last 2-3 tiles to their enemies like Zealots in StarCraft 2. This would fit the Japanese and help them against archer UU.
The only bigger changes should be implemented for Chinese. The skill gap is a substantial problem und stems from their start which should be changed. The latest PUP enhances their extra pop from TCs, perhaps that’s an indication that such a change might come.
Also they have no HC although they invented gunpowder. As their Chu-Ko-Nu is already strong vs infantry, giving them HC should not break them. And there is a nice suggestion about giving them a predecessor of the HC in Castle Age called Fire Lancer: