The key candidates in '60 had been Kennedy, Humphrey, Johnson and arguably Stevenson. With Kennedy and Johnson out of the running, Humphrey's a strong heir apparent and I can see Johnson giving him his full support, which'd be enough to cruise through. Stevenson's Ambassador to the UN so he's pretty covered even ignoring his health issues. Without them, I see Terry Sanford, Eugene McCarthy or George Smathers emerging as possibilities. RFK wasn't ready to go for the presidency yet, IMO.
The Republican field could widen a bit, but I think it's largely similar, besides Goldwater picking a different vice presidential candidate, probably someone who'd pander to the moderate base. If Goldwater doesn't win the nomination, Nixon'll be back for another shot, but I don't see Lodge or Rockefeller making it.