Robert Mueller is illegitimate. Between the leaks and hiring of Clinton donors, unless he starts opening investigations into the Clintons, Comey, and Lynch - there's unlikely anything he does will have any effect.
He already had all the power he needed, so if he's impaneling a Grand Jury, it most likely means he's got nothing. Could we end up with a "Monica" incident (because I don't believe Trump would be any better on the stand than Clinton, but who knows)? Perhaps. Impeached. Hahahaha! Of course, we'd get President Pence, so it wouldn't be all bad.