Reogns vs Brock is likely, but not cemented. A month or two ago Meltzer reported that WWE might've changed plans to Cena vs Reigns and Stroman vs Lesnar at Mania, he doesnt know what the fukk is going on.
If he's not in the title picture I'd see him vs Seth or AJ. I don't think anyone else but HHH/Taker/Cena is a big enough opponent for him, and those guys probably won't make sense as opponents.