I doubt it will be Gephardt. Gephardt is a tired face in the Democratic Party who brings nothing to the ticket. His lackluster presidential campaign last year generated no excitement at all. About the only thing he could do for Kerry is keep Missouri in play and that's not much. Gephardt is a throwback and would do for Kerry what Jack Kemp did for Bob Dole - absolutely nothing.
As for Hillary, are you kidding? That would be a gift from God because Hillary would make Kerry toxic and unelectable in at least 40 states. We'd be talking margins (for Bush) of 50+ points in many Southern and Western states. A Kerry/Hillary ticket would result in a Bush landslide of epic proportions. That is why it would never happen in real life (though I do daydream about the possibility).
"About the only thing he could do for Kerry is keep Missouri in play...."
Gephardt is not really all that popular here in his home state.