I feel like his trajectory is to use more and more antiquated and offensive language to refer to Black people. So my guess is that some kind of old-timey POC slur is where he will be heading next.
"I was named after my uncle Joe! We called him iron Joe because.... When my.... Well when my ma first got a clothes iron, the family... [Incomprehensible], and that's how COVID worked, jack!"
He mumbles his words that makes it sound like he's saying a slur. so he holds a press conference denying he said the slur only to accidentally say the slur at the press conference.
I'll take the dark horse and say it'll be like a black eye or popped blood vessels in his eye again. So not really a gaffe, but more of a nail in the coffin
Biden will be walking along and then turn around with drool dripping from his chin and start talking to nobody and one of his minders will quickly come up to wipe his chin and biden will start yelling at them and drop some slurs.