Guess I’ll speculate.
Potential roles for John Cena:
Ganondorf- they’d likely want someone with Cena’s physique to play the role, and him being older would contrast well with a smaller and younger Link.
Goron- depends on the story being adapted if any specific one, but I could see him fitting the Goron voice and motion capture quite well. Recall that Wes Ball, the movie’s director, more than 10 years ago said that if he ever got to make a Zelda movie, he’d want it to have the Avatar treatment for the fantastical species, so I would expect that here.
King of Hyrule- Daphnes and Rhoam were both quite the physical specimens, so why not? Perhaps that I would imagine they’ll keep the royal family speaking with British accents, which I’m not sure Cena would be a fit for.