Biden I think is too old. But if he had run last year he would have had my vote. Almost any other Democrat besides Clinton against Trump would have gotten my vote. I do like the idea of Kasich, Rubio also. But as you stated, I think it is too early to speculate. If I were a democrat I would look for a fresh face, say from flyover country. Not from the northeast. Running ultra liberals from the northeast has worked out well for the Democrats. Clinton, NY, Kerry, Mass, Dukakis, Mass. all lost. Who won? Obama Illinois, Bill Clinton Arkansas, Jimmy Carter, Georgia, LBJ Texas.
JFK who won in 1960, from Mass was as much as a conservative back then as a liberal. The Northeast was also Republican liberal, Rockefeller Republican country also. Both parties much closer ideological wise without today's huge divide. Both parties had their conservative and liberal wings. Times change.
Will Trump finish his first term? I don't know. He has much to learn, but seems unwilling to do so. The key word is unwilling, not can't, but won't. One thing is for sure, if in 2020 it is another Hillary/Trump match up, I voting third party once more.