I think he needs a character/identity/persona more so than a gimmick, although "gimmick" is an interesting choice of words.
The term itself conjures up multiple passe images we’ve seen over the decades.
A gimmick used to refer to the role a wrestler portrayed: a plumber, a corrections officer, a cowboy, etc.
It could also describe the prop that wrestler used: a nightstick, a whip or lasso, etc.
Then in the attitude era, Vince moved his company away from the gimmick thing, dissolved the line between heel and face, and everybody became more of a personality – the same thing guys like Flair had been doing for years.
That, to me, is the time honored/tested protocol.
And the ones who could do it well never had to rely on clown makeup or a rooster outfit.
Cena seems too confined to his “gimmick.”
He needs to expand his character beyond that one-dimensional paradigm and cement himself as JOHN CENA instead of “that white rapper guy.” But before he can just be John Cena, he needs to “create” JOHN CENA. You do that by making the fans interested in WHO you are – not because you carry a nightstick or microphone.