There is a significant amount of damaging information out there that will find its way into Mueller's report. Some highlights:
1) In the Roger Stone indictment, we are told that "“a senior Trump Campaign official was directed to contact Stone about any additional releases and what other damaging information” WikiLeaks might have. There is likely quite a bit more on this topic yet to be revealed. For example the identity of the person who directed the senior campaign official to do this.
2) We have court transcripts in the Manafartov case indicating that he shared internal polling data that the campaign paid quite a bit of money for with Kilimnik, with instructions on who to pass it on to in Russia. Given that Gates was at this meeting, it is likely we will learn more about this.
3) We have details on the negotiations for a lucrative Moscow real estate deal pursued through June 2016 (at least) on Trump's behalf by Sater and Cohen. Given that Sater and Cohen are cooperating witnesses, there is likely more to learn about this as well.
4) We have the Special Counsel office telling various judges about the cooperation by Gates, Cohen and Flynn on matters that are at the heart of their investigation. The details of their cooperation will come out in due course, but it is difficult to imagine that they would have been offered the leniency they are receiving without providing fairly significant information. Presumably Flynn has provided information on the instructions he was given for his phone calls with Kislyak.
5) There are a number of cooperating witnesses whose contributions remain murky. Nader, Patten, and others.
6) Let's not forget Peter W Smith, who sought out Hillary's emails on the dark web and raised several hundred thousand dollars for "scholarships for Russian students." Mueller has been investigating his activities.
The above relate to collaboration between the campaign (proper and improper) and the Russians. Then there are a whole raft of issues related to obstruction and abuse of power: firing of Comey, dangling of pardons, collaborating with witnesses to release misleading, untrue or incomplete testimony.