Trump could have won the Democratic nomination with his trade and foreign policy positions if he hadn't done the Birther schtick. He'd have to have run a different kind of campaign, however, and it would have had to be in a year where the Democrats felt a bit hard-pressed to win (e. g. 2004).
Trump could have filled the Sanders niche if he hadn't done the birther stuff, kept his old social issue positions (abortion, gay marriage, guns, plus single payer) and toned down the appeals to racial resentment to genuine wolf whistles for the working class northern guys who voted for Sanders rather than the transparently obvious stuff he does now. People forget he had publicly supported Democrats for some time prior to Obama. No way does he win as a Democrat, though (which is the one and only reason he decided to run as a Republican)
