I'm going to go out on a limb here and make a prediction:
It will be Trump vs. Sanders after the primaries.
It's not just that Sanders has the momentum (look at his crowd sizes and enthusiasm, not just the polls. More on that in a moment).
Hillary is imploding with a possible trifecta of felonies arising from server-gate. True, it's mostly the conservative Right touting this now, but now the FBI has gotten involved and there's a tough judge who's even threatened her lawyer with contempt charges, while the State Dept stupidly thumbs their nose at the judge and tries to throw Hilary under the bus. Under the bus is going to get a lot more crowded because many of those officials knew what was going on and condoned it while Hilary had her own server (I am using Hilary's first name in keeping with the media custom to prevent her being confused with her arguably more famous and favorably thought of ex-husband president). Even if Hilary survives the legal challenges - and Bill Still, among others, suggests Joe Biden is being kept in the wings as the replacement candidate if she doesn't - her unfavorables keep climbing into the red levels.
It needn't be stressed that so far, no other Democratic candidate comes close to Sanders in the #2 spot. Expect money and polling to force some of them out fairly soon. Unlike the Republicans, the minority-polling Democrats don't have the Koch Brothers and Sheldon Adleson to finance a losing campaign.
Trump? Well, he's surprisingly progressive, though not on his main issue illegal immigrants, but his plan for dealing with that will satisfy his base of low-educated white conservatives who blame immigrants for taking their jobs. Ironically, even though Trump is firmly in the rentier class, he can rightly claim to be independent, if for no other reason than he is simply too rich to be bribed (NYC's former Mayor Bloomberg had the same claim made about him).
Sanders is generating support from the all-important independents and dissatisfied Democrats who might just stay home rather than vote for Hilary. It was these groups -- including a healthy dollop of young voters -- who came out for Obama overwhelmingly in 2008. If they see new progressive ideas from this 73-year socialist, they will overlook ageism and over-used labels, and vote for him. Since nearly 50% of the electorate does not vote even in the presidential elections -- one of the lowest turnouts among developed nations, and even low by U.S. historical standards -- getting even a small number of them to vote, especially in the primaries would make a big difference (Full Disclosure: I recently switched from Independent to Democrat just so I could vote for Sanders in the NY primary). Sanders seems to realize this with his "southern strategy" of campaigning for Democratic primary voters, as Rachel Madow pointed out in her recent report. Sanders is drawing thousands in crowds, while Clinton makes passable speeches and policy announcements that barely garner any press (in contrast to her email/server issues, which are gathering momentum).
It will be interesting to see an ex-New Yorker (Sanders) debate current New Yorker Trump on a national stage. Even though there is only 4 years of age in between them, Sanders will have to be careful not to be tarred with an "old socialist idea man" label, while Trump will have to shed his oligarch image and reputation as an intellectual lightweight to succeed. Both of these men are tough experienced in-fighters so I wouldn't put it past them to do both. Should be an interesting contest"