Has it been considered to to give cities a secondary battle terrain?
I think that would be thematic and favouring offense a little more over defense. Although I initially though that rebalancing might be a nightmare, I now think that only a few changes would be needed.
Consider a city surrounded by grassland: Some slow-moving spiders can freely approach the city from quite far away (especially if there is a road), while the city's cavalry garrison cannot make a sortie to defend the city out in the field. Therefore the cavalry should receive its open terrain bonus (or maybe half of it) to defend the city.
Vice versa, consider a grassland city defended by spiders and some cavalries attacking it: the cavalries might not take the city, but they could easily lay siege to it. However, in warbarons, you need to spread out on 12 squares in order to lay siege to city, which is impossible (plus vectoring), so one could argue that the cavalries should get their open terrain bonus (or maybe half of it) to attack the city.o
A positive side-effect would be that heavy Infantry becomes a less obvious choice for defending a city. Light cavalries would still be slightly weaker for defense, but suddenly your garrison has an option for offense elsewhere, which heavy infantries would not have. Orcs become a viable alternative for attacking or defending a swamp city. Elves can defend a forest city, andso. So we would see more terrain-fitting city defenders.
Rebalancing could be simple if terrain bonus is halved for cities; or alternatively, initially some units are given a negative city bonus (e.g. -2 for light cav's in city might be sensible).