That does sound nice. I can't recall anything from vanilla that does border expansion specifically, but then again, I think I only ever tried playing vanilla once, so I'm definitely not the best person to ask.Well if it makes you feel any better I'm experimenting with a new improvement called a Frontier Camp. It uses the barbarian encampment art and is constructed by Gatherers. It's a weaker version of the fort and can be constructed outside borders. It gives a culture bomb when built. I also gave this ability to Forts as well. The game moves quick enough and the Gatherer goes obsolete before this improvement can be abused. This way you can snag a nearby resource early on to get your game going. This does not culture bomb neighboring civs though. Just unclaimed tiles. I don't think it's too unrealistic. I'm sure tribes have always been territorial. Are there any vanilla buildings that increase border expansion? Nothing is coming mind.
Is there maybe some way you could give me the mod in its current state? My feedback would be more useful then.