They could start by prosecuting Clapper for perjuring himself to Congress. Unfortunately Trump seems to have zero interest in cleaning out the CIA, FBI, DIA and the rest. Nobody ever gets fired, let alone prosecuted.
Maybe this is another Trump chess move. In exchange for immunity Assange could be negotiated into dumping everything. The US might then put Assange into witness protection. But the accusation gives Trump a place to start. As long as Assange is in that embassy the US can probably do nothing but try Assange in absentia.