We can say a couple things about it. First: Expectations should be tempered, especially given Mueller’s by-the-book track record, his reluctance to testify, and his promise that he won’t talk about anything beyond what’s in the report. (“I would not provide information beyond that which is already public in any appearance before Congress,” Mueller said in May.) And second: That doesn’t mean we won’t learn anything.

Below are some key questions Mueller could answer and subjects he could clarify.

1. What does he really think about Barr?

Barr clearly misled about what was in Mueller’s report, and he did it flagrantly enough that Mueller took the unusual step of writing him a letter about it. Even after Mueller laid out extensive evidence and explicitly said he couldn’t clear Trump of the crime of obstruction of justice, Barr reached a wholly different conclusion, saying he wouldn’t accuse Trump of it.

It seems unlikely Mueller will air disagreements from within the Justice Department, and we should hardly expect him to repudiate Barr publicly. But he could make his feelings known more subtly, including by making statements that contradict what Barr has said – as he did in his public statement in May – or by talking about the things that led him to write his letter.

2. Would he have charged Trump if not for Justice Department policy?

Related to the point above, the Justice Department has a policy (in the form of an Office of Legal Counsel opinion) against indicting a sitting president. Barr has said Mueller said privately that “he was not saying that, but for the OLC opinion, he would have found obstruction.”

That’s confusing. The impression left is that Mueller was saying the evidence wasn’t compelling enough, but Mueller’s report told a different story. It seems possible that Mueller was simply saying he didn’t even try to reach such a conclusion, because it wasn’t his job.

If that’s true, it would render Barr’s statement pretty meaningless – and further indicate that he was spinning Mueller’s report in a pro-Trump direction. Some clarity on that would seem warranted.

3. Why didn’t he press to interview Trump and Donald Trump Jr.?

Obstruction charges require evidence that the alleged perpetrator intended to obstruct legal proceedings. Despite that, Mueller relied only on written answers from President Donald Trump and other evidence to establish his frame of mind. His report indicates that, even though he regarded Trump’s written responses as “inadequate,” he decided that a protracted subpoena fight over an interview with Trump would have delayed the report too long.

Mueller didn’t interview Trump’s eldest son, Donald Trump Jr., either, even though Trump Jr. organized a Trump Tower meeting with a Russian lawyer promising damaging information about Hillary Clinton. Mueller said that he considered charges against Trump Jr. but that he didn’t believe the government could prove he acted “willfully.”

4. No collusion vs. no conspiracy

The focus on obstruction and the Barr-Mueller drama has obscured another very valid issue: the distinction between the report’s conclusion that there was no conspiracy with Russia, and Trump and Barr’s claim that Mueller found “no collusion.”

Mueller says at the start of Volume I of his report that nothing the Trump campaign did rose to the level of a crime, but he also made clear he was evaluating the narrow legal concept of “conspiracy” rather than the broader (and nonlegal) concept of collusion. In fact, the report says the Trump campaign welcomed Russia’s help and “expected it would benefit electorally from information stolen and released through Russian efforts.”

With the right line of questioning, Democrats on the committee could really dig into how enthusiastic the Trump campaign was to accept this help. Mueller needn’t even step outside what’s in the report, but instead focus on the actual evidence and reinforce the distinction between conspiracy and collusion.

5. Another possible obstruction angle?

In a recently released voice mail, Trump lawyer John Dowd probes Michael Flynn’s lawyer for tips about what Flynn might share with Mueller’s team after cutting a deal to cooperate with them. Dowd also alludes to Trump’s warm feelings about Flynn – which, when heard in the actual audio, carries clear overtones that point to a potential pardon.

The voice mail would seem to present a compelling avenue of inquiry for Mueller – especially if it delivered a message that Trump himself authorized. The voice mail could have figured into the obstruction investigation, and some have argued that it sounds a lot like witness tampering. It’s somewhat similar to other potentially obstructive episodes Mueller detailed in depth – most notably, Trump’s conduct toward former campaign chairman Paul Manafort, which included publicly leaving open the possibility of a pardon.

Mueller’s report says his team did not interview Dowd “because of attorney-client privilege issues.” As some legal experts have noted, though, that privilege doesn’t extend to illegal conduct.

6. The investigation’s origin story

One frustrating aspect of the Mueller report is that it doesn’t deal in-depth with the so-called “Steele dossier” – that document containing unverified allegations from British spy Christopher Steele that was used to get a FISA warrant to monitor former Trump campaign adviser Carter Page. That dossier has become central to a GOP conspiracy theory, now entertained by Barr, that there was something untoward about the origins of the Russia investigation.

We shouldn’t expect Mueller to launch some extensive defense of the investigation’s origins, especially given the FBI launched it about 10 months before he was appointed as special counsel. But he might be able to tell us how much he focused on the document and how it was regarded within law enforcement. Really anything he can say about the what he learned about how the investigation was conducted before he came on board could be important for the public record, because this is now the subject of its own investigation.