Is there a good case for Barkley, Kamara, and Mixon over DJ and Bell in dynasty? Likely depends on how much you value security and track record vs. youth and longevity. There's an argument that the ppg is going be similar for these guys moving forward, which would obviously favor the younger backs. Bell and DJ are entering that age range where they're no longer going to give you maximum roster flexibility and trade value since some owners are not going to play ball for a 26-27 y/o RB. With the strong opening week for the younger guys, the window may have already closed.
Looking around the NFL and trying to rank the RBs by current overall talent level, this would probably be my A tier in no particular order:
Mixon
Bell
Zeke
Barkley
Kamara
Gurley
DJ
Fringe: Fournette, CMC, Hunt, Gordon
Next tier down on raw talent level would probably include Fournette, D Freeman, CMC, Hunt, Gordon, Howard, Chubb, Drake, Hyde, McCoy (aging), Cook, Ingram, and Guice with Penny, R Freeman, Kerryon, McKinnon, L Miller, Ajayi, and Henry as fringe options. I would define that tier as "starter worthy, but not elite." After that the list becomes a mixture of pure committee backs, opportunity JAGs, and career backups.