niemann is not the next right handed 2nd coming of randy johnson. back when he was 17-0 for Rice, he had a 99 mph heater. he doesn't have that anymore and never will. atleast he hasn't gone for tommy john like his Rice teammates, Humber and Townsend.
I think if Niemann can command and get ahead of the count with his fastball between 91-94 to both parts of the plate, while throwing his killer curveball as a strikeout pitch, and mix it up with his two other pitches, while leveraging his height - then he can be a very good major league pitcher in the next two years. Despite his height, I don't see him as an overpowering pitcher. Granted, he throws harder than Mark Hendrickson, but he's not RJ.
He'll be a good pitcher (because he's not just a flamethrower), IF, and a big IF, if he can stay healthy. Young pitchers rarely do, so I wouldn't count on anything in the near future. But if you have patience, he's minor-league keeper worthy, and may break tampa's atrocious rotation next year. Edwin Jackson, I think because he was the guy they got for Baez, and not because he's more talented than Niemann, gets first shot this year.