Joe Biden's problem
Trump isn't going anywhere. He's demonstrated time and again that he's a petulant man who holds grudges.
I expect that he will fume on the sidelines, and that over the next four years we'll see a steady barrage of Trump tweets needling and insulting the Dems and Biden, jumping on every misstep Biden makes. He will be a major thorn in the side of the current administration.
If America is lucky though maybe he'll surprise them and settle into a quiet life of golfing and KFC. Or maybe he'll be too tied up with lawsuits now that he's no longer protected by his office and will direct his attention elsewhere.