Mueller impaneled a grand jury. In most situations, this means he's justifying expenditures, possibly getting vengeance, or pursuing a legacy. Whether this is what's happening will only be known in the future.
There's another possibility: Due to the political nature of his appointment, and his knowledge of how damaging his opinion can be, his decision might be to absolve him of any political motivation, and allow what's considered the people's right. He can present all the evidence he's accumulated, give it to the grand jury, and since the evidence is so tenuous, if not non-existent, the final decision of whether Trump, or any of his cohort, did something that demands a warrant will finally rest with a panel of citizens charged with determining whether an indictment is required.
Time will tell, but the entire event is tainted with political motivation, and an unwillingness of too many to corrupt the liberties demanded by the Constitution. The guidelines established by the Constitution were followed, and Trump was elected by the correct paths to follow. This irritated many, showed a weakness of perception by those too willing to think they are special, and led to some of the most seditious efforts to destroy a government founded on principals of liberty.