I've had fun with the idea of the Democrats trap-dooring Biden out of the nomination at the last minute, but in the cold light of day I'm confident he will be the nominee. The train wreck will go forward as pre-ordained.
Nor will he put Hillary Clinton or Michelle Obama on the ticket as his running mate. The worst-case scenario of Trump winning re-election is bad enough without giving him a landslide (or a bigger one than he would already get), and neither former First Lady is going to want to be blamed for a defeat of those proportions.
The longshot odds of a Dem victory won't keep the party from leaning on Biden to choose somebody youthful and promising, just in case -- somebody they'll convince themselves (wrongly of course) is better than a Dan Quayle or a Sarah Palin. Stacey Abrams might fit the bill, now that Andrew Gillum is off the table, or they might find a far-left Latina who isn't disqualified by age (like AOC) or by being even more scatterbrained than Biden (also like AOC).
So no, there is no Black Swan Event looming on the blue horizon. Trump will defeat Biden in November and that's that.