It’s a little dangerous to speculate about the unknown unknowns. There may be aspects of the investigation, either in the final report or farmed out to other agencies, that aren’t even on the public’s radar. In fact, it seems likely that there are. So we should expect to be surprised.
With that caveat aside, it appears that Mueller’s report focused fairly narrowly on whether the Trump campaign coordinated with Russia in its electoral interference operation, and whether Trump attempted to obstruct the investigation. But we also know that, in the course of pulling on those threads, Mueller’s probe hit on numerous other issues he farmed out to other offices — such as the violations of campaign finance law to which Michael Cohen has already pled guilty, as well as Flynn and Manafort’s status as unregistered foreign agents.
We’ve had hints of many other topics that Mueller’s office probed that wouldn’t necessarily relate directly to those central questions: Whether foreign governments illicitly funneled contributions through Trump’s inaugural committee; whether the Trump Organization was engaged in money laundering or other fraudulent transactions on behalf of Russian oligarchs; whether foreign money was illegally channeled through the National Rifle Association. And we know from court documents that there are multiple ongoing investigations that arose from the special counsel’s inquiry, but are now separate from it. So it seems entirely possible that the biggest “bombshells” to come out of the Mueller probe won’t be found in the report at all, but instead relate to ancillary crimes that other offices are currently investigating.
This was, at least originally, both a criminal and a counterintelligence investigation. And there are a number of questions left unresolved that don’t necessarily relate to criminal conduct, but are highly significant from a counterintelligence or national security perspective. One question is whether Russia had some form of “kompromat” or other leverage over Trump — which might help to explain why they were so eager to promote Trump above other Republican contenders. That could be true without Trump having committed any crime (or any crime directly related to the election) but it would obviously be quite urgent for the public to know about.
There are also questions about the campaign’s stance toward Russian interference that go beyond direct “coordination” or “conspiracy.” We know the campaign was eager to make use of hacked Democratic e‑mails, and that Trump and his surrogates sought to cast doubt on Russia’s responsibility for that intrusion long after the Intelligence Community considered these to be established facts. If the Trump campaign knew or believed that Russia was indeed responsible at the time, that might not be criminal, and it might not amount to “conspiracy” — but it would be extremely damning all the same.
It does sometimes seem as though nothing short of an actual felony matters in contemporary American politics. But there’s a wide array of conduct Mueller might have uncovered — “complicity” as opposed to “collusion,” let’s say — that you’d hope would be disqualifying for high office, whether or not it amounts to a literal crime.