Regarding styling, I think we can assume there will be elements of the Avenir in future Buick designs:
http://www.buick.com/avenir-concept-vehicle.html
In theory they could just drop a bigger engine, beef up the suspension, and offer a 6 speed manual in a lower trim Avenir and call it a GN. But since there's a new Regal on the way, it will likely be under that nameplate/model.