Its too soon to say. It depends where Trump's presidency is. The Party is not going to waste an up and comer like Booker, Gillibrand, or Harris on a failed campaign against an incumbent. Especially considering how thin our bench is.
If midterms show that the public is responding to Trump positively and it looks like he will get a second term, expect someone like Kaine, Cuomo, O'Malley, or Hickenlooper. Basically, a party elder who wouldn't be viable in a year without an incumbent who will be given a shot just for the sake of having someone challenge the incumbent (think along the lines of Bob Dole).
This may also be the circumstance in which the Democrats let the then 80 year old Bernie Sanders run. Make no mistake, Sanders will NEVER be President. If you thought Trump beat Clinton badly, imagine what he would've done to Bernie Sanders. Bernie Sanders is a man who has, on the record, praised Fidel Castro and Che Guevara (bye bye Latino voters). He is a self-avowed socialist. It may be unfair, but that is still a very dirty word in America. Especially among the type of voters who gave Trump the White House. And none of that speaks to the fact that opposition research on Sanders was never released (and trust me, its out there...his ties to Silicon Valley V.C.s makes Clinton's ties to Wall Street look pure and innocent). Trump would've had a field day. And being as Sanders has no composure, Trump would've destroyed him in the debates. But if its looking like Trump is an unbeatable incumbent, the Party may clear the way to give Sanders the nod just to shut him (and his supporters) up while also dipping their toes in the pool of lite-socialism and seeing how viable it is.
If its looking like there is an outside, albeit unlikely, chance of unseating Trump (let's say he is in a position like Bush in 2004, a fairly strong incumbent but not unbeatable), expect to see someone like Sherrod Brown, Elizabeth Warren or Al Franken. These are the high risk, high reward candidates. Brown is pretty far left (probably the most leftist member of the Democratic Party) but has a very common-man appeal. Plus he is a rust belt Senator who has fought against free trade and has a lot of appeal to Trump voters. Of course, being just shy of socialism could burn him in a big way. Franken is in the same boat. He is slightly less left than Brown but would have a lot of trouble being taken seriously. And Warren...well...its sad to say, but if you thought Clinton got slack for being an "unlikable" woman, imagine the kind of hell Elizabeth Warren will get. Plus, she is pretty far left. The reason I keep coming back to that is that most Americans are uncomfortable when you go too far on either end of the political spectrum. Any of these candidates COULD conceivably beat Trump. But it would be an uphill battle. They will be used if it looks like there is an opportunity, but a small one.
Finally, if Trump is a weak incumbent or decides not to seek a second term, that is when you will see the likes of Booker, Gillibrand, Harris, Klobuchar, Castro and probably some of the other people I already mentioned. In this scenario I like Gillibrand or Booker combined with Sherrod Brown as the running mate.