Title asks the question really.
Seth Rollins would be a good choice (probably the best one). He is a member of the authority, who Daniel Bryan has had run ins with for over a year now so it would be fitting for him to face Rollins, who is the authority's number one guy. But I get the feeling Rollins will be occupied with Orton at Mania.
Beyond that Rusev would be another smart choice. Although I'd much prefer if Reigns (or Ziggler) got this spot and was the one to defeat Rusev, it wouldn't surprise me if they chose Bryan to be the one who does it to give one of their top stars a "Wrestlemania moment". This is a match I think could work well to some degree. The big man vs small man nature of the match would make it an intriguing battle and I think both performers are capable of pulling it off. The only problem would be that it would feel like "Bryan vs Brock lite".
I think Cena would be another guy that could bring a big match spectacle to a match with Bryan. Although they have faced each other before, a match at Mania between the two so called biggest faces in the company could be entertaining and they do have decent chemistry together. It's a match The Authority could put it together as a way of trying to get the two guys that have defied them the most against one another. Why not have the two guys that The Authority has tried more to screw over more go at it at Mania? I would probably prefer Ziggler in this role more than Bryan (especially if they gave Ziggler the victory) but it wouldn't surprise me if they booked this to give both Cena and Bryan an prominent position on the card outside of the title match.
Either way, I am hoping that none of these are even an option and that we do get Brock vs Bryan for the title at Mania!