John Cena has come a long way in the world of wrestling and it's mainly because of his charisma. His ability to make audiences laugh and to appeal to them is uncanny, and that is essentially what the Shazam character represents. It's a character that doesn’t take itself too seriously and the movie likely will have a lighter tone than other DC films. Cena has shown he can bring the humor when he needs too while still maintaining a healthy dose of intimidation.
Cena definitely has the physical makeup to play the character, the studio won’t have to worry about hiring trainers for this guy. Shazam is a character that could go toe-to-toe with Superman, so you have to have a bulked up actor that could compare to the Man of Steel himself. Cena also has the athletic ability to do his own stunts and the mental capacity to learn these stunts quickly. He would be more than physically prepared for the role.
1) Acting Experience:
Cena has had many movie roles and his acting ability isn’t the greatest but that still fits well with the Shazam character. Which essentially evolves around a kid’s mind being put in a powerhouse of a body. So the lines that Cena would have to deliver would not be overly sophisticated. It should fall well within his acting range and something he should nail. Also audiences aren’t coming into a Shazam movie looking for tantalizing dialogue but for knock-down action which Cena can easily deliver on.