Mueller has already given Comey blanket immunity for any and all of his crimes. Comey is untouchable now.
How does he have authority to do that?
I've pieced together some things from various media reports on this FISA surveillance (and all the implications of it) over the last nine months, and I'm starting to develop a wild, outlandish theory about where this might all be headed. Grassley may be dropping some hints about it, based on the report earlier today where he has asked the FBI to explain if they ever warned the Trump campaign about possible attempts by the Russians to compromise them ... the implication being that the "investigation" of Trump associates like Manafort will not be seen as credible if they never warned Trump at all.