Guild-owned cities - Count me in on this notion
. It'd be great to have supersized stronghold as like the "capital" for your guild. This could be the place where you trade with others in your guild without having to actually go to each individual's home city. Also, I think that each player should have the option of stationing some of their own troops at the city, as part of the garrison. This way, they would get a share of the loot after a successful defense. There's a dozen other potential features.