Brennan briefed Obama about Hillary's Russian Collusion plot
What's important about this new Spygate bombshell is that Brennan found out about this plot to smear the Trump campaign with false accusations of Russian collusion from Russian sources (demonstrating the Russians knew of it, and thus could "helpfully" supply false accusations) and then briefed Obama (and presumably VP Biden) about it, demonstrating that Obama's decision to order the FBI to continue to investigate those false accusations was made while provably aware that the accusations were groundless.
In fact, the Intelligence Community allegedly referred the allegations about Clinton's Russian Collusion scheme to the FBI for investigation on September 7th, 2016, well before the election.
That's not "Hey, maybe the Trump campaign is colluding with Russia"; it's "Hey, the Clinton campaign is trying to spread false allegations that the Trump campaign is colluding with Russia".
And yet, the FBI already had an investigation open based on those false accusations and continued to pursue it, even well past the election.
If this claim by Brennan and the intel community can be proven, it would seem to put the FBI on the hot seat for knowingly pursuing false allegations against the Trump campaign and then the Trump administration.
UPDATE: Probably also worth noting specifically. When something is "referred to the FBI", the usual context appears to be referring a potentially criminal act for the FBI to investigate. So if Clinton's attempt to falsely link the Trump campaign to Russia is being referred to the FBI, it's not being referred because the FBI should investigate whether Trump is linked to the FBI (as actually happened); what the FBI should be investigating is whether Clinton is trying to frame her political opponent.