I like the idea of an additional greco-roman faction. I don't know of any historically, but including something on, say, the italian peninsula such as Etruscans would be cool. Or perhaps another barbarian faction. or not. whatever.

Not to make a comparison, but I liked the Illyrians in RTR6. They were underpowered and faced huge threats, consequently getting destroyed every time, but I liked their irrelevance and obscurity. It was fascinating.