I'm hoping Chris Jericho signs back with WWE and challenges him for the United States Championship. Mister Hands brought up a great point in a thread I created a few days ago on what Jericho could do at Wrestlemania should he make it back in time, which was that Jericho, with all of his accomplishments, has never held the US Title. Simple set-up, could even open the show (and possibly steal it).
If that's off the table, I think I'd like to see Jack Swagger jump to Raw in the build-up to Wrestlemania to feud with Bryan over the United States Championship. The All American American chasing the US Championship, it writes itself, and the match would be good, as the Old School Raw showed us. Again, it would be an ideal opener at Wrestlemania.
Don't see Cody Rhodes being an option, since he's probably set to wrestle Mysterio at Wrestlemania.
However, if they actually go with William Regal, I'd be very happy. I don't see it happening myself, but if it did, I'd have a huge grin on my face.