With Ferrari announcing that Sebastian Vettel will not be returning to the Scuderia for 2021, where will the four-times world champion end up?

As I doubt he will be wanting to play second fiddle to either Hamilton or Verstappen, Mercedes and RBR seem out of the question (unless Lewis himself changes to Ferrari). Also, I don't think he will want to go to another Ferrari-powered team like Haas or Alfa Romeo.
McLaren (with Mercedes engines in 2021) might be a possibility, as does the Aston Martin works team or Renault if Ricciardo leaves.
Or is it back to the beginning with Alpha Tauri?

In lack of a competitive drive, Seb might also choose to retire or to take a sabbatical.