Assuming the expanded rosters allow teams to carry more than 13 pitchers, the most obvious area to take advantage of additional roster spots would be on the pitching staff.
If the season starts in early June, theoretically, the Angels would have a healthy Shohei Ohtani ready to take the mound. They could also see the return of starter Griffin Canning. That said, both pitchers will most certainly be on strict innings limitations, as they would have been anyway coming back from injury, and especially following what will probably be an abbreviated second Spring Training period.
Joe Maddon will need extra middle relief men who can get outs early in games while saving the back of his bullpen for high leverage situations. I would expect that if the starting rotation turns into Andrew Heaney, Julio Teheran, and Dylan Bundy, along with Ohtani and Ganning, we could see starters Patrick Sandoval and Matt Andriese as candidates to fill the extra roster spots.
This would allow the Halos to use a 6-man rotation during the first month of the season as the starters build up length in their arms. Andriese gives the coaching staff options as a reliever, spot starter, or long relief man. He entered camp looking to make the starting rotation and pitched well during the spring, allowing only one run over nine innings of work.
The Angels recently optioned several pitchers to the minors who could suddenly find themselves back with the big club under expanded rosters. Most notably, left-handed reliever Ryan Buchter, whose send down was a bit of a surprise to begin with. Jose Quijada is another option from the group of players recently optioned. Between Buchter and Quijada, it would seem to make sense to carry at least one of them on the active roster, as they are the two primary lefties for an otherwise right-handed heavy bullpen.