While the revelation of a secret meeting involving Robert Mueller in the delivery of uranium to the Russians, by itself, does not prove anything of a criminal or unethical nature, it does raise questions that merit an investigation.
After all, when Mueller was FBI director under the Obama administration, he was trusted enough by Secretary of State Hillary Clinton to carry out this mission. If it was a diplomatic mission, why was the FBI director involved?
And if it was a law enforcement mission, why was Clinton involved?
The one thing Barr should not do is go after Mueller in any investigation. This will be portrayed as Trump seeking revenge and the optics will look like hell. Barr should start with Weissman, Strozk and Paige, Ohr, Brennan, Baker, Comey and FISA.
Pull a Manafort on every one of them and connect the dots. That should lead back to Mueller where he will be forced to make a deal to out Clinton and Obama. Under no circumstances do you go after Clinton or Obama directly. The goal is to have the underlings point the finger at them.