3. David Peralta
I said that you shouldn't be surprised by this name. While Peralta is still a good fielder out in left field, he's 33 years old and will be slowing down soon. The Diamondbacks are overwhelmed with options for the outfield, excluding the precious Ketel Marte.
Peralta's bat struggled a bit last year, although that could be due to just how bad the team was. Additionally, the long season might have begun to take its toll on him towards the end. Regardless, he still performed in the outfield rather well with a 1.2 UZR and 0.7 Range. It'd be wise to keep him healthy so we can keep his bat in the lineup. Thus, perhaps 2-3 games a week he starts at DH.
This will allow the young outfielders like Jake McCarthy, Pavin Smith, Daulton Varsho, and others to continue to play. It'll allow Peralta's leadership and bat to stay involved too. The former Silver Slugger winner is in his last year of his contract with the team. Let him end it on a strong note!
The last and final DH candidate in my opinion is a young player that proved his versatility last year.