Robert Mueller is a fair and very competent man, has a stellar history from his High School, College and U.S. Marine Corps service. I trust him to do a stellar and fully complete review of all that is known today, and to uncover any that is not yet fully known, to record it impeccably and to determine his commission's future, whether it turns from Special Counsel, to Special Prosecutor.
First, once he finds office space and furniture, and hires his own trusted hands, investigators and lawyers, to which they have allocated 50 days and unspecified amount of money (unstated to date that I can find), he can go to the beginning and investigate which Russians or which others tampered with the U.S. Election and/or if anyone tampered at all. That will of course require seizure, forensics on the DNC servers and computers from their offices that were used during the Campaign.
Same with RNC, I would expect, to gain a full understanding. I imagine Seth Rich will come up in his review, and WikiLeaks, et. al., and will be interesting to see where all that goes. Presuming he finds evidence of Russians, or whoever else bugging or hacking John Podesta's group, Donna Brazille, and D.W. Schultz et al, he can see where that leads, see the level of involvement of President Trump, and others in his sphere, and follow the fact where they may lead.
Lots of depositions to follow, and follow the trail where it leads, without any reservations. There is no time limit on a Special Counsel, and if he uncovers illegality in other areas along the way, that falls into his providence as well.