Why not Carter's running mate, Walter Mondale? He's a Midwestern (Minnesotan) Senator who sat in the center of the Democratic tent, was a dear friend to the late liberal Dem champion Hubert Humphrey, and seems to have had positive relationships with the other factions of the Democratic coalition (minorities, blue collar union types, academic progressives, and the Southern Dems like Carter himself alike). IIRC Mondale's status as a potential unifier with appeal to every corner of the party was a reason as to why Carter, being keenly aware that more progressive Democrats were leery of a Southerner like himself heading the ticket, chose him to be the 1976 VP candidate in the first place.
Alternatively among the options you posted, Carey would be a pretty interesting choice as well. Man managed NYC's economic crisis in the mid-'70s and seems to have been quite fiscally conservative for a late 20th century Democrat (slashing taxes, reducing spending, trying to induce growth with tax credits) if his record as Governor of NYS is any indicator. Considering his socially conservative shift (at least on abortion) toward the end of the '80s, and that a Ford '76 victory would keep the Rockefeller Republicans relevant for longer, a two-term Carey presidency spanning the '80s in Reagan's place might also have a much longer-term echo in the form of the Democratic Party evolving in a more populistic, conservative direction while the Republicans adopt a libertarian approach to social issues as well as economics.