I actually think we should do something like this. Maybe this will be a good way to make wasteland more useful as well.
Cities should still be able to build all types to at least lv4 though. If cities can only build Lv4 SFs, it would cripple some realms.
I'd go one step further and make rurals produce better stats for cavalries (or give bonuses when you build the RC maybe. +10 to all stats or something).
But yeah generally, I think having regions outside of cities being able to upgrade RCs to lv4 would be a nice idea.
