Prosecutors investigating a scheme involving multiple potential defendants almost start with the small fish first and work their way up to the capo. So far, Mueller has bagged a bunch of Russians (who will never be prosecuted here), small fish Papadopulous, van der Zwaan and Pinedo, larger fish Butina, Flynn, Gates, Manafort and Cohen; and Roger Stone will be tried for charges arising from his alleged involvement with Wilkileaks in November if he doesn't plead first. Witnesses who have spoken with investigators include Alan Weisselberg, who probably knows even more than Cohen. When Stone was busted, the FBI seized an enormous amount of data from his home and devices, and Flynn, Gates and Cohen are still cooperating.
This means there must be more evidence still being collected and evaluated that was not included in Mueller's report - because Barr stopped the OSC investigation. The reason the report didn't find evidence of "collusion" or a conspiracy with the Russian government (at least according to Barr's letter, which he probably wrote three weeks ago before he even saw what Mueller had discovered) was because Mueller was shut down before he could find enough of it to warrant more indictments, particularly those of Trump Jr. and Kushner. What he'd have done if he'd found evidence that Trump himself was involved still isn't clear.
The investigations are continuing with other prosecutors, and if there is prosecutable evidence of "collusion" (more accurately, conspiracy) on the part of any of the big fish, it will eventually be found, but it will not have the political impact of Mueller's report, and in the meantime Trump is free to twist and poison the media narrative - which is what he wanted all along.
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