First, he could name anyone nominated to any other position who has been confirmed by the Senate to serve as the acting attorney general.
Yes, that means, Nikki Haley — now serving as the US ambassador to the United Nations — could be named acting attorney general. Or any other cabinet member. Or some of the few lower-level confirmed members of the Trump administration — like David Bernhardt, confirmed just Monday as the deputy secretary of the Interior Department.
Additionally, and opening up a potentially much more broad universe, Trump also could name anyone at the Justice Department who makes roughly $100,000 a year or more (Grade 15 on the General Schedule pay scale, or “GS-15,” or higher) and has been there for 90 days in the past year to serve as acting attorney general.