Judging by an interview I just watched (video below) it sounds like a fair amount will be added.
Some of the things in the interview worth noting including "8-ish" characters, a hint at online lobbies and bonus stages. It'll be priced at the same price as an "update" would but just on a physical disc which, to me, means it'll be about the same price as GTA IV DLC. I think we'll get more than he mentioned as he claims they considered DLC but it wasn't "practical". I'm interested and there is a high chance I'll pick it up but I need more info before it becomes a must.