Assuming that he doesn't get caught up in any scandals or dies, then I'd say JD Vance is so far the obvious favourite for the Republican candidacy at the next US election. However it's still very early days and you never know if or when a dark horse candidates comes in out of nowhere and just takes over, much like Trump himself did in 2016.
Regardless of who it is though, I think it will almost certainly be another Trumpist as I struggle to see a future where the corporate Republicans make a comeback, unless Trump seriously fucks up this term and leaves the US as a whole worse off than it was under Biden. But that is a seriously low bar to reach given how chaotic the Biden administration was due to Joe's cognitive incapacity as well as the extent of corruption within the Democrat party.
As for the Dems, it's way too early to tell as they're still licking their wounds and cannibalising one another for power following last year's defeat. Their big problem is that there currently isn't really anyone viable for a leadership position that doesn't have a history of being either horrifically corrupt and incompetent, like Gavin Newsom, or batshit insane, like AOC.