The key known unknown, of course, is whether Mueller will find that Trump committed an indictable offense. The intermediate questions are as follows:
1. How far back will Mueller go? There is a credible argument that Trump has been operating as a Russian intelligence asset for nearly thirty years. Will Mueller attempt to put Trump’s behavior as a candidate, and as president, in the context of his business dealings, or not?
2. What did Mueller get from his plea agreements? Did anyone tie Trump personally to the Russians? We don’t know yet.
3. What, if anything, happens with Cohen? By telling the world that he values his family over Trump, Cohen sent a message that he expects to be compensated for his silence. Trump undoubtedly understood that. How will Mueller respond to that?
4. Will the report discuss a lower burden of proof? On obstruction of justice, for example, it is perfectly possible that Mueller could find that a preponderance of the evidence supported the allegation. That would not lead to an indictment, but it would be relevant to impeachment. Will he discuss that, or just reach the conclusion that he would not indict, and leave it at that?
5. Will the report address more political questions? Impeachment is primarily a political–not a legal–proceeding. You can imagine a scenario in which Mueller finds credible evidence of “collusion” with the Russian government, but cannot tie it to any criminal statute. Would he address that issue in the report? I would argue that he must, but it remains to be seen.