What can Mueller do?
The answer is nothing.
He can tie up staff and principal advisers in very expensive legal demands and responses. In addition to money, this diverts their time and attention from the Trump agenda. He can leak to the media damaging information. He can get his Grand Jury to indict key staff. He can get warrants including predawn raids like he conducted on Paul Manafort at his house. He can load the legal gun for Ryan and McConnell to impeach and try and convict Trump. And who knows what other trouble a politicized, highly motivated legal staff with deep ties to the Uniparty state can come up with.