Third party candidate
I agree that this is a historically good time for a third party candidate to have a chance of success, with so many republicans refusing to vote for Trump, and many democrats who are unexcited, at best, about Hillary.
The problem is the prospective third party candidate, not being beholden to party orthodoxy, would support some democratic ideas, some republican ideas, and some ideas that neither party supports. The parties will use the things the third party candidate agrees with them on to paint it as "stealing votes from our side and guarantee the evil [Clinton|Trump] wins easily", and use the things neither party supports to paint the third party candidate and "dangerous and/or crazy".
This worked with Perot, after all, though the climate now should increase resistance in both parties (but especially the republican party) to being told what to do by the party establishment.
I think the main chance of success is that republicans who don't want Trump obviously don't want Clinton but probably view four years of her as a guarantee that a republican candidate will win in 2020. 12 years of one party is about as much as this country can stand, historically. However, if Trump wins they're stuck with him as candidate in 2020 and have the worry he causes a permanent fracture in the party. So a fair number of republicans might not worry if a third party candidate caused Clinton to win, thus a moderately conservative third party candidate might have the best chance here. Who that is, I have no idea.