Yeah he genuinely checks a lot of boxes to the point it's kinda surprising he's not a more popular fancast.
-He's the perfect age (26)
-He's a rising star that's bound to become super big eventually (the Oscar nom is just the start)
-His look, while perhaps a little unconventional compared to previous Supermen, is also not completely outside the norm to the point that the image of him in the suit couldn't become iconic.
-He's by all accounts an extremely strong actor that could probably bring quite a bit to the role.
It also helps that in a lot of ways he feels like the anti-Cavill. And I don't know... in a lot of ways he feels like he could pull off something maybe a bit closer to Golden Age Superman or maybe the Tim Sale Superman.
If Gunn were to do an extremely traditional take on the character in the sense of almost being Christopher Reeve but modernized I feel Corenswet would be the obvious choice but if Gunn is planning on putting some sort of twist to the character idk... Mescal feels like the pick he'd go towards.
He also feels similar to Will Poulter getting cast as Adam Warlock.