Biden refusing to concede and supporting an electoral challenge after losing to Trump.
This is underrated honestly especially if the race is very close. I don't think they will do something on Jan 6 but I could see his team meeting with electors to throw it to the house and try to select a non-Trump Republican rather than reinstate Biden.
More likely Democrats will just parrot their favorite lines since forever (i.e., "election interference! voter suppression!") as the rationale for challenging Trump's legal win. Of course, the mainstream media and red avatar mafia will put in overtime saying It's (D)ifferent!
How serious it gets depends on how close the results are otherwise. Trump's win coming down to less than 5k-10k votes in a single state is close enough that a D House may actually reject his victory.