Would it change the situation for the better if cities dealt less damage but were more resilient? Archery units would have a tough time attacking cities because of the increased HP, while melee units would still be effective and wouldn't get punished as bad because of the reduced damage.
I'm all about reducing the static power of cities and modifying that balance in some way. They are just so oppressive when it comes to maneuvering, especially when trying to probe your way forward against tightly packed cities.
My idea for a while has been archer's attacks against cities hit the garrisoned unit first as a buffer to damaging the city. The garrisoned unit should heal either 10hp per turn if it moved/attacked or 25 if it fortifies and receive a defense bonus (that scales up with oligarchy and that pantheon belief you cant ever get on higher difficulty). [Note: my hp numbers have not gone through balance testing] Siege and melee attacks target the city straight up just like they should, and because cities are less powerful themselves and less devastating they can actually start to do damage in conjunction with your archers fighting the garrison.
Some neat things about this idea are if you're under siege and have a swordsman with cover II he might actually be better placed in the city if there are a lot of ranged attackers or if you need him to sally to attack battering rams or another adjacent unit.
Also it takes power away from the almighty city and gives it to the standing army, allowing for strategic play. On the defense, killing ranged first or the last melee unit is so cut and dried right now. You will see a true strategy dynamic form from having to decide whats better to target, kill off the archers to spare your garrison or kill off the meat and siege to save the city. More battles will revolve around keeping a city able to receive fresh troops and not getting surrounded, because if you can't keep a healthy garrison or sacrifice the occasional unit, your city will take double damage when the archers start hitting true.
There would and should be dynamics beyond cities being the wrath of the gods on the "battlefield." I mention a city on the battlefield with some contempt, as they are far too strong. Militaries should be what determine outcomes, not just some proven turtle social policy and tech path.
In my opinion this whole post is part of one huge suggestion to buff melee units and soften range.
Edit: Just one other thought for right now: Medics next to cities = a high value target that makes a lot of sense when extrapolated. They could contribute more significantly to city sieges/defense strategies than ever before, giving power to the units and taking it away from the dumb space fortresses that are cities.