He’s probably angling for the VP slot. He can’t defeat Hillary but with his unparalled tax and spend record, many Dems would find him an attractive running mate. He is unquestionably farther to the left than Hillary so the party might tap him to balance out the ticket.
That said, this ticket would leave the Dems with two northeasterners so that could prevent this particular “dream team” from coming together.
The next VP is going to be a hispanic. Bet on Julian Castro.
The dynamics of 2016 will be different from those in 2008 or 2012. I don't know if Hillary qualifies as a "pure" Northeasterner, as she grew up around Chicago, and lived for about 20 years in Arkansas until she moved to Washington, DC and later New York. However, she is old, female, and white, so it is possible if not likely that her Vice Presidential running mate will be none, or just one, of the above. Julian Castro is a possibility, although he would not carry his home state. If Jason Carter, Jimmy Carter's grandson, is elected governor of Georgia, he would become another possibility plus possibly win Georgia for the Democrats for the first time since his grandfather was elected in 1976.