Not sure how Mueller would be a witness, though. He was an outsider when nominated, presumably. Unless Comey also confided in him.
Also, it's possible that some people could argue that his nomination for Special Counsel was tainted, but he's a Republican (right?) and has just appointed democrats for his team and has a good reputation.
The main issue is who is capable of supervising his investigation in the DOJ. And I mean allocating funds and resources.
At some point, I wonder if recusal by Rosenstein and Sessions is enough.