Morris Udall is certainly possible, though improbable. No sitting U.S. Representative has been nominated for President since 1880 (Garfield), after all. He's also from Arizona, which has a less-than-favorable record when it comes to electing Presidents.
Jerry Brown could take the nomination if he enters earlier, but he's pretty young and if elected, would be the youngest POTUS in American history. Nice crossover appeal to Republicans, though (with his flat tax proposals and fiscal conservatism).
Henry M. Jackson is IMHO, the most likely non-Carter nominee. Hawkish (to make up for the less-than-stellar dove candidacy of McGovern), an old school, New Deal Democrat (guarantees support of the white working class), environmental and ERA credentials (to reach out to the Democratic-voting New Left).
George Wallace is highly unlikely. He's going to have to rebuild bridges with the African-American community ASAP if he wants to even have a slight chance at taking the nomination. If Ford is the nominee, he probably becomes the first Republican since Hoover to win a majority of the African-American vote.
Frank Church is a possibility. Pro-gun, anti-war, generally liberal. George McGovern with an image that's hard to tar and feather, essentially.
Lloyd Bentsen is going to scare off liberal voters.
Robert Byrd ditto the Wallace and Bentsen comments.
Sargent Shriver may be a Kennedy-relative, but he's got next to no experience in elected office. And he was the Veep nominee in '72. That's not a very good track record.
Fred Harris is going to scare off moderate to conservative voters.
Birch Bayh is a possibility in the same way that Church and Jackson are.
Hubert Humphrey is a possibility in a deadlocked convention. If he makes more than a half-hearted attempt to win the nomination, he might be able to clinch it.
So, in my opinion, your best bets are either Jackson, Bayh, or Humphrey.