Scott Pruitt. He's a tough guy who will be willing to fire Mueller and his motley crew of Democrat lawyers.
That's the reason Swamp RINO's like Prissy Lindsey Graham are trying to warn Trump off of moving Sessions to DHS. They know if Trump moves Sessions to DHS and Pruitt to DOJ as new Attorney General, that Mueller's investigation is now in serious jeopardy since Pruitt can take oversight of it from NeverTrumper Rosenstein and do it himself (he wont be recusing).
The Swamp wont have any leverage to hold over Trump's head if say AG Pruitt cans Mueller and blows up the probe or even ends their fishing expeditions and keeps them restricted to the Special Counsel instructions to keep investigation limited to the 2016 election and Russian Collusion ONLY. Also might well remove/replace Mueller for cause for DOJ conflict of interest violations for NOT recusing himself while overseeing an investigation involving family or friends (Comey).