To fix large scale issues, it is not uncommon to revamp the backend even if the UI stays the same. eBay went through a few rounds as its user base grow.
On top of that, the operators usually need to add new services, UI and extensions to stay competitive or expand their business. Since John Schappert is keynoting in GDC 2008, I'd imagine he's going to show us the next XBL lap there.
No one has really delivered on C2C commerce yet, and then there's Game 3.0 to counter, plus clan management, better Internet integration and tighter integration with other MS services and products. A few people have mentioned that XBL (current version) is not the only way to do online gaming and digital downloads.
Above these technical changes, what would be more interesting is the end user value (i.e., What "package" and economic benefits, if any, will people get on the new XBL)