Mike Flynn, Trump's former National Security Advisor, may be the next domino to fall. Remember Trump kept Flynn on for weeks even after he knew Flynn was a foreign agent with ties to Russia. Trump only fired Flynn AFTER the newspapers broke the story, and AFTER Trump fired the Assistant Attorney General who told Trump and Pence about Flynn.

How is Mueller going to get Flynn? That's a good story.

His chief deputy is Andrew Weissmann. Weissmann headed the Enron task force, the team of prosecutors charged with investigating the financial collapse of the huge energy corporation. Weissmann and the other Enron prosecutors wanted the cooperation of Andrew Fastow, Enron’s former chief financial officer.

When he charged Fastow, he also indicted a new defendant: Lea Fastow, Andrew’s wife, who had also worked at Enron. With the felony charges pending against Lea Fastow, the couple faced the prospect of spending years in prison while their two young sons were raised by others.

Fastow caved and agreed to testify against the others in Enron.

Mike Flynn's son worked with his father’s lobbying firm and was also involved in the Trump transition. Almost any prosecutor would likely agree that using legitimate charges against him as leverage against his father is fair game.

How Robert Mueller can play hardball with Michael Flynn - The Washington Post