Trump shows up.

And singularly attempts to override the format of a debate. He tries to make it a televised campaign rally by continuously shouting over the moderators and Biden, knowing that CNN and the broadcasting networks won't cut away. Hey, it's entertainment AND history in the making.

You’d have to scream like a banshee to be heard over Joe’s mic.

but when his mic is on, it will be obvious that his mind has gone out to lunch somewhere else.

He will not answer a single question, he'll just riff and babble and be incapable of finishing a single sentence. His fan club will proclaim it the best performance of his life. Everybody else will be appalled.

So it's a combination of #1 and #3.