This isn't my prediction, but I'm starting to like Tom Perez as VP.
I've liked him for VP for a while now. He seems like a good fit in terms of helping appease liberals but also a wink and a nod to Hispanics.
The other choices seem unpalatable - Castro has virtually no experience and he isn't nearly ready to handle the duties as POTUS, should that come. Kaine would have to give up his VA Senate seat, which is not safe in a 2017 off-year election. Warren has numerous issues that many have already written about, and Brown would endanger any possible Senate majority. Perez just seems like a good middle ground here.