I can see Fury (if he's mentally healthy, motivated and on steroids) giving trouble to Usyk because of his size and awkwardness and Klitschko because of his jab and wrestling. Haye would be a dangerous fight for Usyk too but other than those three I think Usyk could beat Ortiz, Joshua, Wilder, Povetkin, Pulev and the rest of the HWs, they match up well with him stylistically.
Usyk is a damn good fighter, he's really impressed me so far, he may very well be a threat to any of the big boys if or when he moves up.
I can't see why he wouldn't be a threat. He's a big cruiserweight and will only get bigger with age. Also his style of boxing doesn't depend that much on size, his speed will be a huge advantage at HW. I'm certain he'll become a champion at HW at some point and build a good legacy there.
Maybe if he unifies and moves up in 2 years. He can start taking on the HW. Now he needs to stay at CW and clear the division