I don’t know what Mueller knows, and he has vastly more experience in this field than I do, so I’m unlikely to second-guess his conclusions, whatever they may be.
Assuming, for purposes of argument, that Trump lets him finish his job, it appears to me that there are four possible outcomes of the investigation:
1. He has Trump indicted. This is very unlikely, as no previous special counsel has tried it, regardless of the strength of the evidence. It is much more likely that the ultimate resolution of any findings of illegality will be left in the hands of Congress.
2. He writes a one page letter saying there is insufficient evidence to support any charges against Trump. The public will not be satisfied with this, so I doubt he’ll do it.
3. He writes a detailed report explaining why, in his opinion, there is insufficient evidence to support any charges against Trump. In all likelihood, there will be ammunition for both parties in this report. Obviously, impeachment will not ensue. The report, if anything, will help the GOP slightly in the election.
4. He writes a detailed report explaining why, in his opinion, there is adequate evidence to support charges against Trump. Democrats will cry for his impeachment, but the GOP won’t bite; the foundation has already been established for an argument that Mueller was biased. The report becomes a huge campaign issue and helps the Democrats.
#3 and #4 are the most likely outcomes. Neither will result in impeachment, but #4, under the right circumstances, could lead to Trump’s resignation.