Given the period, maybe Sal headed west out to San Francisco, or working in LA. More likely he's working off-Broadway as a scenic designer or costumer. Or maybe he'll turn up working on spec for Peggy.

He could be directing commercials in NYC and runs into either Harry or Megan. Megan, not knowing his connection to SCDP (and she works under her maiden name so he wouldn't know she's also Mrs Don Draper), brings him home for dinner one evening, lol. Imagine the pillow talk between Don and Megan that night.

I'm guessing we will see Sal briefly (a la Paul Kinsey) either directing a commercial in NYC that Megan stars in, or possibly in the last season at Stonewall, which was June 1969.  I hope they do a crawl over the final credits at the end of the series, like "American Graffiti" which tells us how each character ended…