If Edward Kennedy runs, it's probably his, but one of the main reasons he ran was because he felt Carter couldn't run the government or deal with Congress (true) and so he might not well run in an ATL where Ford or Reagan wins in '76.
Mondale was well liked and Jerry Brown wasn't as kooky in '80—they're both strong possibilities—but it might be too early for Glenn (and he'd need a different campaign staff).
A number of '76 Democrats would also consider a second try, like Scoop Jackson and Mo' Udall (can you imagine a truly funny President?).