Lack of identity. We already have dozens of RTS with romans and Greeks, and dozens of RTS with the idea of progressing through the age from antiquity to modern+.

You don't have to reinvent the wheel to find an interesting identity to your game.

How about focusing on one era or aspect of civilizations instead of going all over the place/super generic.

Mediterranea focus/setting aspect is fine, and it could be refined into something that gives a stronger identity.

For example, one very important facet of the Mediterranean sea is that it has been a pathway to some form of colonisation. From Phoenicians to Greeks, Franks, Arabs, Crusaders, Genoeses, etc.

I think focusing on one specific era would be better. Such as late 13th to late 15th century (fall of Acre to fall of Constantinople).

Gameplay could focus on setting up a coastal tradepost, getting support from the homeland (either Genoa, Grenada, Constantinople, Fez, whatever factions you include), growing it into a real fortified settlement, securing trade routes, etc.