This is a list AFTER the Big 4 are either gone or fading fast... Silly moi, thought that was obvious... anyway... How Nole reboots will affect their prospects in a big way... but, 2020, that's the time frame for the list, sort of...
Raonic and Dimitrov seem sort of interchangeable, with respect to potential... with the caveat that Milos has progressed a bit further... Milos without his injury proclivity could well be #3... should be... but, sadly he's really hampered... I think Milos might well get a major at the either W or USO, so, as I wrote above, he could be #4...
Not sure about Coric, but, he seems a one surface type guy... but he's so young, he's still deep into his development phase... and Tiafoe is an ATP baby, so, no one knows...
P