While the outcome of the 1984 election was a blowout, the pre-election polling for much of the year had Mondale in striking range. Indeed, as I recall, Reagan's poll numbers got somehat shaky after his bad debate performance as the "age issue" became salient.
Mondale's campaign blundered in a few places: the acceptance speech tax hike pledge and the Ferraro pick were probably the two biggest. If one were to change both, Mondale would have a fighting chance. The Ferraro pick really hurt because it created weeks of stories about her and her husband's finances. Another thing that hurt Mondale was the drawn out nature of the primary campaign, which did Mondale little good and in fact hurt because of the unfavorable contrast in some respects with Gary Hart. Taking Hart out of the primary campaign early would probably help.
So, if you negate those factors, have Mondale pick someone like Bentsen or Glenn for VP (Glenn would have had to improve his dismal primary campaign somewhat) that do his campaign no harm and perhaps broaden its appeal a bit (the flaw with the Ferraro pick was that it appealed to voters he already had in the bag), and have Reagan bobble the second debate in the same manner as the first, you could have a scenario where he ekes out a win. I don't think under even the most generous scenarios it would be a big win, but a narrow one is not completely inconceivable to me.