Haven't seen her name mentioned yet in veep discussions, but wonder if Kirsten Gillibrand might not be a bad choice. She was a Blue Dog while representing a conservative House district, but since her ascension to the Senate, has moved solidly in a progressive direction.
Many of her most public stances (DADT, 9/11 first responders, military sexual assault) have engendered broad appeal and belie a strong feminist underpinning, and her voting record and personal politics all suggest support for progressive causes. Of course, I think it would be a stretch to imply that she would be politically Bernie Sanders-esque as VP, but she's experienced, well-respected, and could be an interesting-if somewhat of a compromise-pick.
More importantly, she lines up well with many of Hillary's personal strengths while bringing in a more charismatic and youthful angle to the campaign that it seems to sorely lack. She's also been a long-standing Hillary supporter, working on her first campaign for Senate, and declining to run for president to challenge her. This pick would have some nice symmetry with that of her husband and Gore in that it would by necessity be 'doubling-down,' both on gender and geography, but I think it would have a different enough feel to avoid further painting her into a Bill 2.0 corner.
In many ways, she's a bit of a blank slate too. She's smartly chosen to lead on a few choice issues as mentioned above while being a bit more of a backbencher and generic Democrat on others. I suspect some of this is strategic and in conjunction with her youth, energy, and general freshness, should allow the campaign to frame her in a mutually beneficial light. To progressives, her public legislative pushes and personal ideology can played up, and for moderate wings of the party and independents, her candor and more mainstream Democratic positions might have more appeal.
Moreover, NY has a Democratic governor and a relatively demographically safe Senate seat, and she's also a candidate with clear higher ambitions for whom VP might wisely be seen as a stepping stone to a presidential run herself.