I know it won't happen, but it'd be wild if the Republican party looked at Biden, said, "we could win this if we didn't field trump" and nominated someone else as their candidate; while the Democrats said, "people aren't going to vote for trump, but they may not vote for Biden either, which could cost us the election" and fielded someone other than Biden. The result being that neither Biden nor Trump end up on the ballot.
Again, it's highly unlikely to occur (though we are living in "Strange Times"), and we're almost guaranteed to see Biden vs Trump, but it'd be interesting to see how that'd shake things up.