Donald Trump could run as an independent as he has threatened to do. It would take a serious organizing effort to get on the ballot as an independent in most states; there is no evidence that Trump has started such an effort which would likely alienate him further from the GOP. Some states, Ohio is one, have laws which might bar him from running. But what if he did?
Don’t get your hopes up, Democrats–depending on who the candidates are, Trump might draw more votes from Democrats than from Republicans. If the Republicans choose a candidate with right-wing appeal, Republicans would have little incentive to turn their backs on that candidate. Whoever the Democratic candidate is, there are sure to be Democrats who would cast a protest vote for Trump.
Then there is the electoral college–local issues might influence Trump’s appeal. In TX or AZ, his views on immigration might be important. Also, consider ethnic groups in certain states; would Trump appeal to ethnic and minority voters disappointed with Republicans or Democrats? Would gender groups, men or women, be influenced?
Trump is unlikely to run as an independent, but if he did it would make the election very interesting.