It's all too early but what about Sherrod Brown/Kamala Harris?
Sherrod Brown comes off as a younger version of Joe Biden without the gaffs, he's also more liberal but not really in your face about it. I think moderate and liberal Democrats could get behind him and he's from Ohio.
Then you have Kamala Harris, while not considered as liberal and seen more likely from the Clinton side of the Democratic Party, she is smart, and brings diversity to the ticket and I cold see her being president one day.
Anyway, big tent party needs big tent candidates.