Just for kicks ... and I wouldn't defend any single ranking. I know this is a crap shoot and I don't really feel too strongly about any of them.
1. Thomas Harley - Close to NHL ready; Significant upside
2. Jason Robertson - NHL ready; Significant upside
3. Ty Dellandrea - Close to NHL ready; Top 9 Center
4. Mavrick Bourque - 3 or 4 years; Significant upside; Easy argument he should be over Dellandrea
5. Jake Oettinger - Close to NHL ready; 1A or 1B in a tandem; He's not Vasilevskiy
6. Joel Kiviranta - NHL Ready, Top 9/Bottom 6; Probably should have been left off
7. Nicholas Caamano - Close to NHL Ready, Top 9/Bottom 6, Probably makes the 2020-21 team if Kiviranta isn't signed as UDFA
8. Ryan Shea - Close to NHL Ready; 3rd pair D; May never play in Dallas outside of injury replacement
9. Jacob Peterson - 1 or 2 years; Top 9; Best European prospect per Takko
10. Fredrik Karlstrom - 1 or 2 years; Top 9/Bottom 6; Great season in SHL right now
11. Riley Damiani - 2 or 3 years; Top 9; More offensive upside than Back, Gardner, or Tufte
12. Oscar Back - 2 or 3 years; Checking center; More upside than Gardner or Tufte
13. Rhett Gardner - Close to NHL ready, Checking Center
14. Riley Tufte - Close to NHL ready/2 years max; Bottom 6
15. Evgeniy Oksentyuk - 2 to 4 years; Offensive upside, Could be career AHLer/return to Europe
16. Adam Mascherin - Probably not an NHLer; Offensive upside
17. Joel L'Esperance - Close to NHL Ready, Less upside than other Top 9/Bottom 6 forwards, Could be a career AHLer (Curtis McKenzie)
18. Daniel Ljungman - Long term Project, Scorer
19. Dawson Barteaux - 3 or 4 years maybe, 3rd pair D, Not a great shot to play in Dallas
20. Jerad Rosburg - 1 or 2 years, 3rd pair D, Not a great shot to play in Dallas
I would be shocked if Harley, Robertson, Dellandrea, Bourque, Oettinger, and Kiviranta play at least a full season or two in Dallas, and many of them are likely long-term assets. Some could be trade options though.
Caamano, Peterson, Karlstrom, Damiani, Back, Gardner, Tufte, and L'Esperance are all going to probably play NHL games as cheap Bottom 6 options sooner rather than later, and it could easily be on a different team. I think Caamano is a lock for a few games with Dallas this year, but I added him to this list because he's a potential expansion target for Seattle.
Shea is probably the most likely to make the NHL of the bunch, but he and Barteaux and Rosburg might have a single shot at an NHL spot in Dallas soon.
I would guess Oksentyuk has the best shot at an NHL career, but I would guess Ljunman has more upside. Mascherin has a make or break season this year in Texas.
The only person I feel guilty leaving off is Antonio Stranges. He seems really exciting. The fact that he doesn't see eye to eye with his OHL coach yet, and he's not putting it together in the OHL yet scares me. Blame it on Scott Glennie, but attitude or potential attitude scares me. He has better upside than several guys I listed ahead of him.