Avengers: Infinity War - 10 Possibilities For The Secret New Character

2. Sentry

If we are genuinely to accept what James Gunn says about Adam Warlock not appearing in the MCU before his appearance in Guardians Of The Galaxy Vol 3, then it stands to reason that the Russos will need someone to take the place of him in the Infinity Saga (assuming it remains at all close to the comics).

It is he who is entrusted with protecting the galaxy from Thanos in the wake of him losing the Infinity Gauntlet, as head of the Infinity Watch, and it wouldn't make sense to task an existing character with that. Aside from Vision, none of them are powerful enough (and he's probably going to be killed off).

It would make far more sense to introduce Sentry using the established comic book convention of Infinity Stone victims being absorbed into the gems (and later freed), before he plays a key role in bringing down Thanos. And you wouldn't even then need to give him his own movie (though that would be excellent also), just have him entrusted with guarding the Stones alongside a new version of the Watch.