I know the game's not out and I haven't played it but I just keep thinking about how each of the building meeples has a different shape (in the deluxe edition they have VERY different shapes) and yet there is no in game use to them having different shapes once they're on the board. Theoretically all the buildings could be disks and it wouldn't effect gameplay.
So my idea for the expansion is to make the shapes matter. That each different building type has some kind of unique function or scoring once they're on the board.
That's it. That's all I've got.
I don't have a while expansion designed yet, but I have a start, and I think you'll be pleased to hear about it:
I have 4 new building types, each with their own effects, but the Cath is that you can only build them where an opponent has built a specific building type.
For example, a Keep would need to be built in a hex with an opponent's Castle, a Chapel is built with an opponent's Church, a Market is built with an opponent's Farm, and I forget offhand what you can build with an opponent's Bank