Better be fill-in, and not a Prequel or Sequel.
Indeed. All they need to do really is to pick something unexplained from a prior episode and go from there.
Like...how did the fly get in? I could write entire story arcs around that, include new characters that come and go entirely because they let the stupid fly in. You could even have them wind up in the desert, gun to their head with a manic Walt wants to kill them for tampering with his operation.