It's never too early, Trump has been campaigning since February.
I'd be surprised if Hillary went for it again, she's gotta be tired. Or exhausted, or dehydrated, or has pneumonia - whatever they claim
. Booker gets mentioned a lot but I just don't think he has a high enough profile (yet). I suppose many felt the same way about Obama though.
I would put my money on Warren - she'll get the Hillary voters that will support
any woman and the Bernie voters that want socialism. I would imagine pretty much anyone who voted for Hillary would vote for Warren and she's in the headlines pretty frequently.