Also since for example Swamp City is 50 Admin, and it's in a SWAMP, perhaps some nations could actually get pretty nice forts in their preferred terrain. Give "nature" based nations better forts in Forest, but remove Fortified City, and give them Fortification, or Ramparts or something in plains. By the same token, boost production somewhat in mountains/caves for those nations who reside there (maybe wastes for Abysia?). Reason being that while the terrain is rugged and inhospitable to most, it is their favored habitat, thus you would assume they've put some thought into the engineering and infrastructure to make industry more lucrative in those terrains.
Also I'd think it wouldn't be game breaking to boost underwater forts a little in production. As it is, it's abnormal to see underwater provinces with >30 resources, and poor little Kelp Fortress is only 10 Admin. To balance that, maybe say non-aquatic races can built fortifications underwater (you can leave them down there for years if they have magic items to breathe), but either disable unit creation other than amphibious and indies, or just let people train troops and have them drown immediately.
And finally, I think that increasing the gold benefit of Admin would go a LONG way to at least somewhat altering the behavior of the players. Making the more costly forts generate a significant amount of gold compared to the more "outpost" style, would make it more of a strategic choice than it currently is now. As it is it seems most DO go only for the cheap fast forts, and if there is anyone else like me, I choose them for location only, regardless of type. Neither behavior places any value on the Admin rating, or any of the actual properties of the fort itself.