I do think that the best candidate for next new big title winner is Jannik Sinner. That said, not sure he can get past the steamroller that is Alcaraz. If the kid wins this tournament, I think we can officially declare the beginning of the Alcaraz Era - he seems to be in that zone of "I can beat anyone," which we haven't really seen recently outside of the Big Three and, for moments, with Andy Murray (in late 2016) and Stan sorta.
What I think he needs to do to solidify his reign is beat Rafa at Roland Garros and Novak at a Slam, whether RG or USO (unlikely at Wimbledon). I know he's beaten both (at Madrid last year) but not on Slams.