The first issue that must be addressed by Mueller is whether any existing criminal statutes would be violated by collusion between a campaign and a foreign government, if such collusion were to be proved? Unless there is a clear violation of an existing criminal statute, there would be no crime.
Obviously if anyone conspired in advance with another to commit a crime – such as hacking the DNC – that would be criminal. But merely seeking to obtain the work product of a prior hack would be no more criminal than a newspaper publishing the work product of thefts such as the Pentagon Papers and the material stolen by Snowden and Manning. Moreover, the emails sent to Trump Jr. say that the dirt peddled by Veselnitskaya came from “official documents.” No mention is made of hacking or other illegal activities. So it is unlikely that attendance at the meeting violated any criminal statute.