Mueller cannot indict Trump. What he can do is prepare articles of impeachment and deliver them to Ryan. They will be about things like the transition team trying to influence the UN vote on Russian sanctions, obstruction of justice for firing Comey and perhaps other process crimes for those surrounding Trump. I would also not be at all surprised if they dredge up some other personal matter about Trump or his business to damage his political viability.
My guess is that Mueller will go after a few underlings and will call it a day with nothing on Trump.
Rosenstein has Mueller on a leash. After watching Rosenstein testify the other day I believe he is ready to redeem himself.