Quoting Fezziwig,
reply 4
I agree that allowing the AI to upgrade them is the best solution. However, I kind of wrote off the idea of the AI being able to upgrade outposts at this point since it has never been implemented even after our repeated requests. If the AI can't do it, I'd rather that the player can't too. Giving a fixed bonus within the zone of control for an outpost seems to be a decent fix.
My idea I suggested many times in the past was instead of individual upgrades , just have outpost levels. That would have been simpler for the AI to handle, and less micro for the player. Maybe 3 levels of outposts.
I tossed that idea out some time ago too. However, it was never really determined as to what the best way was to obtain those levels. Experience seems like the obvious choice because of the other mechanics in the game....but how? Obviously not populations. Perhaps turns....though that would mean there is no way to speed up leveling?
I would recommend 5 levels to the outpost.....keep it in line with cities. 