I've been thinking this through, and I think the best possible candidate the Democrats could nominate in 2020 is Bob Casey.
1. He understands the WWC voters: He has always done well with those white working class voters in Pennsylvania that Trump won over to carry the state in 2016. His base is also Coal Country in Northeast Pennsylvania, and he could help win back a lot of Rust Belt and Coal Country votes.
2. He represents what will likely be the most important state in 2020: Pennsylvania, with its Even or R+1 PVI and its 20 electoral votes, will be the most hotly contested presidential battleground in 2020. And, assuming he wins reelection in 2018, he would be in a good position to take back the state for the Democrats.
3. He is somewhat pro-life: One of Hillary Clinton's biggest weaknesses this year was here support of a radical policy on abortion. I think that, if she did not unapologetically defend late-term abortion and advocate repealing the Hyde Amendment, she would have won more votes of suburban conservatives wary about Trump, but who could not vote for that radical of a position on abortion. He won't flip the most hardcore pro-lifers, but he could get some of these aforementioned voters (which are plentiful in potentially competitive states like Pennsylvania, Wisconsin, North Carolina, Georgia, and Florida) with his pro-life with exceptions stance.
4. But, at the same time, liberals can't call him a social conservative: Other than on certain nuances of abortion and guns, Casey largely toes the liberal "party" line on social and economic issues. Despite opposing abortion, he actually supports Planned Parenthood. He's generally pretty liberal except on the few issues that have turned off so many voters to the Democratic Party.
But, hey, go ahead and ignore my advice, saying that I'm a Republican who doesn't know what's best for the Democrats. I think he is your best candidate.
I think he would be a good VP choice, not sure about how effective he would be leading the ticket though.