I think Senator Reed in Rhode Island might be a good candidate. He could definitely excite the net-roots (lifetime ADA rating of 97) without scaring anyone else. As opposed to someone like Warren, he isn't as obnoxious about his views and in 2008, he managed to win about a quarter of all McCain voters. Also, his seat isn't up in 2016 so there would be no worry about giving up his seat to run. Lastly, he might be a decent candidate to win over working class voters given the state he represents.
Jack Reed is a complete scumbag. Terrible, terrible choice. It'll be John Edwards all over.
If we're talking about Democrats, Hickenlooper might be misunderestimated.