#3 As a contender at Money in the Bank
Remember how Cena was a participant at both Elimination Chamber and Fastlane? This is because he still remains a top draw for WWE to this day. Could he be brought on board to add star power and sell tickets for Money in the Bank? Don't expect him to win the briefcase though!
This is an unexciting possibility but it is certainly a probable one. Having Cena in this match could actually spark off a brand new feud based on how the match actually pans out.