With Paul Ryan having pointed out that Mueller is a solid Republican, I wouldn't be surprised if he straight up ordered the relevant committee chairs to appoint their own special investigator, one who's answerable to Congress and not the executive. That person could probably use Mueller's work, too; Mueller is using Preet Bharara's, after all. (Hell, Congress might decide to stick it to Trump and choose Mueller.)
Trump's alienating the only people standing between himself and impeachment (Ryan and McConnell) and it's hilarious to watch.