I guess you can use any criteria you want for 'best player'.
If best player has any links to impact on winning basketball games I think there are a few more people on this list. Mind you, DM's potential to rise up this list is real.
Currently 'better' than Mitchell (no particular order) and in ADDITION to the ones you listed (Minus Beal):
Khawi
Covington
Simmons
Cousins
Draymond
Gobert
Jokic
Lowry
Otto Porter
KAT
Kemba
George (despite crappy latter part of season)
Conley
Jrue
Hayward
I think he is in the same ballpark already as the players listed below - probably have him at the top of this list and projects to be better than all of them within 5 minutes in anycase.
Porzingis (?)
Love
Blake Griffin
Deandre Jordan
Ingles
Klay Thompson
Marc Gasol
Beal
Eric Gordon
Wall
Rubio
Middleton
Van Fleet
Devin Booker
Horford
Olynyk
Garry Harris
Tobias
And ahead of these guys:
Wiggins
Saric
Nurkic
Roberson
Adams
Capela
Dragic
CJ
Jamal Murray
Kuzma
Tatum
Dennis Smith Jnr (easily)
Schroder
Ingram
Randle
Ball
Harrison Barnes
Hood
So all in all I have him at 31.
I found it interesting that on WINS he is ranked 32. And many of the people I had ahead of him were ahead of him on Win Shares. Not sure how great that stat is, but it matched with my 'eye' test and glance at other stats. The only players I had ahead of him that were not ahead of him on WINS were Kyrie and Rudy.
I estimate him to be top 15 within 18 months.