The next steps would be ordering the justice department to prosecute him, going to court, and appealing all the way to the new Supreme Court so they can overturn the precedent. Which would require either moving very quickly or preventing the other side from taking power, one way or the other.
Of course, by then pandora's box is open. As long as someone is willing to follow those kinds of orders, nothing would prevent the next president from doing the same thing. It's a slippery slope not unlike the one that caused Rome to go from being a republic that viewed regicide as a fundamental virtue to an empire that would persecute groups for denying the divinity of the emperor.