Someone mentioned it earlier, but Biden's a gaffe machine and his record will scare a lot of progressives away. I also fear he might have a scandal or two in the cut.
Beto might as well run for president in 2020. This is the highest his profile has ever been. Might as well strike while the iron is hot, raise his profile even further either win the primary or secure a VP/cabinet spot with however the Dem primary winner ends up being.
Also surprised how little Sanders has been mentioned in this thread so far. Sanders has the best shot at the moment imho (still the most popular politician in the country) and has been putting in a lot of work in the past two years, with no sign of stopping. His VP pick will be a huge factor due to his age.
Sanders/Harris is a strong ticket that should garner support across all sections of the Dem party. Same with Sanders/Gillibrand. These tickets may have some issues in the general, but I think it will be enough to win.
Sanders/Warren would be very well liked within most of the left-wing of the party, but might have issues in the general due to be. Warren will also be 71 in 2020, so that will hurt said ticket.
Sanders/Brown is a "safe" ticket overall, but two old white dudes? Might be an issue to a lot of non-white voters. Together, however, they have a strong chance in the Midwest and will be competitive regardless.
Sanders/Beto is a ticket with massive potential, depending on Beto. If he continues the momentum he currently has, I think this can be a winning ticket. Complements Bernie's age as well via being much younger than him.
Sanders/Gabbard will be very well liked by the "Bernie" wing of the party, but will have issues with a lot of mainstream dems due to Gabbard's former conservative views from a decade ago and her stance on Syria. However, I can see this ticket winning with many lefties, independents, and some repubs in part, again, due to Gabbard. She also complements Bernie's age by being a lot younger than him. A huge high risk-high reward ticket.