It's an idea I really like, and than I also asked a while ago. I would like it for balance reasons and for flavor reasons too.

The flavor reasons are subjectives : I would like to see generic slingers in ancient era, and generic archers in classical era for exemple. (archers being only an ancient unit). And ranged militia looks for me as a good opportunity to represent them, and they will always be a weak unit, just a bit more viable, because able to attack from their defensive positions (city walls) without being highly damaged.

The way of implementation : I personally like the current design of militia, in term of artworks and models, so I am not sure I would like than the basic defensive units (from ancient to early modern era) directly get ranged attacks. So I maybe prefer the idea of Scotsman : An infrastructure improvement.

Another idea would be a repeatable improvement.

Another idea, but maybe odd and  too complex : If the attacker is waiting several turns to get more siege engines and decreasing the number of  defender militia in the city, maybe the defender militia should gradually get better as they are decreasing while they are under siege ? So they get some veterancy and become ranged, the whole being justified by some civilians training tech ?

Because so far, if I don't have any reinforcement nearby, I am personally never waiting some turns to make a sortie. I make it most of the time directly, so the attacker is not able to have siege engines, and it is easier to cheese the Ai with more numerous militia.

With this system, maybe it will be more interesting to wait a bit before making a sortie and give a layer of depth about waiting as besieged player ? Like, waiting for having two ranged militia, instead of four melee for exemple.