Of the ATP players currently ranked in the top-20, I'd say:
Felix, Khach, de Minaur, Ruud, Bublik, Baez
My list is probably smaller than most, mainly because I worry about how injury-prone guys like Sinner and Alcaraz might end up being. And Djoker eventually has to tail off. So I think that there will be opportunities for guys like Tsitsipas, and even Zverev in the near future.
I cannot see writing off a guy like Rune. He's barely 21 years old and has already made a 3 QFs. Where was Federer at this age? Through his age-21 season, Federer had 2 career QFs in slams, which includes 0 during his age-21 season in 2002(includes a Wimbledon 1st round exit in straights by a qualifier).