This is how WWE should handle a Cena turn if they ever decide to go down that road.
For those unfamiliar, Johnny Gargano was becoming a John Cena like superhero babyface in Dragon Gate USA. Overcoming ridiculous odds, holding the title forever (something like 600 days now), white meat babyface hero to the point it was on the verge of becoming annoying.
So he has this classic match at WrestleMania weekend with Shingo, one of the top stars of Dragon Gate in Japan, and a guy who hasn't been to America in like 3 or 4 years. Watch the video and see what happens.
Basically his heel antics ruined a great match with a special opponent, with the story being he couldn't put Shingo away to win otherwise, so now Gargano is doing a mock up/lack of self awareness white meat babyface deal as a heel. It's actually pretty brilliant and has pumped new life into what was becoming a pretty stale scene in DGUSA.
My point here, is Cena would be perfect in the "heel who has no idea he's a heel" wink wink role.