Gibson and McKissic are locks. If the Washington Football Team decides to keep just three backs, as they did last season, then I think, as boring as it seems, that back will be Barber. With Curtis Samuel able to run the ball and with a good blocking tight end/H back likely to make the squad, Barber’s skill set may be the best fit for the team.
Either Miller or Patterson could force his way onto the roster as pure lead back depth, so that McKissic does not have to take on that role, should Gibson be slowed by injury. But Patterson would seem most likely headed for the practice squad. Miller will have to prove he can still be productive in order to stick around. Williams would seem to be here primarily as deep depth in the event of injury. As he showed back in 2018, he is a good fit in that kind of role.
We did not consider Curtis Samuel in this assessment, but he will almost certainly run a few plays a game out of the backfield. We also are not considering how much the addition of the more mobile left tackle Charles Leno Jr. will benefit the running game, but you can read about that in my upcoming piece on the offensive line.
Those factors, combined with the natural maturity of Antonio Gibson, should provide the Washington Football team with a very solid running back corps in 2021.