It'll be some sacrificial lamb, some absolute nobody who filed paperwork wrong. Mueller has to justify his existence and expenditure somehow, so he'll charge some petty bureaucrat and point to him screeching "LOOK LOOK I'M DOING SOMETHING! SEE?". Otherwise people are going to start wondering why tens of thousands of man-hours and millions of dollars are being thrown at investigating the party we now know is innocent.
>Obama (but let's be serious here Presidents operate with total immunity)
If he's not a sitting president he doesn't have the same legal status. A sitting president has to be tried by the senate, but a former president is just a regular citizen as far as the law is concerned. That being said in practical terms every democrat alive would jump on a live grenade for the man, as he is their living icon of progressiveness.