This means he has already landed the role in the Marvel movies or Cena is actively campaigning for the role.
I think it is possible but not likely. Cena has the physique and has acted in about a dozen movies. He is 40. If Marvel cast a new actor for Captain America, then I think they would go for a younger actor.
They could keep Chris Evans with some extended contract. Chris Evans in 37.
They could keep Steve Rogers, Captain America character out for a few movies.
Winters Soldier and Falcon are in a Disney streaming series. The main Captain America related actors could be shifted out of movies for 2 to 3 years. The focus could be Dr. Strange and Black Panther and other characters.
If Marvel is switching actors for Steve Rogers, I think they younger 30 something actor. They would also probably pick someone less famous and less expensive. The recasting of Spiderman seems to be instructive of how they would recast Steve Rogers. Spiderman is a younger character but they went with a less famous actor.