Donald Trump could not have won a Democratic primary given all of the racist, sexist, homophobic, xenophobic, Islamophobic, etc. crapola he's spouted. But, to indulge the OP with the hypotheticals, in this scenario I would pull a jfern and be #ReadyForJillStein.
If Trump were running in the Democratic Primary, he'd have dredged up the Clinton scandals and moved the Democrats to repudiate Clintonism in the same way he moved the GOP to repudiate Bushism.
If Trump were running in the Democratic Primary, he'd have been pro-choice, he'd have let his social liberalism be on display, and he'd have picked a female running mate already.
The problem for Trump would be that he already declared himself as a Republican in 2012, was a birther, etc. If he hadn't done that, however, and had run as a Democrat, he'd have toned down his insults because the crowd he'd be pandering to would be less receptive to that. (The GOP eats up Trump's insults; he does them with more panache and gusto than the half-baked hacks who've been dishing them out for so long.)