I think that one of the takeaways from last week’s election was that Trump’s victories in Pennsylvania and Michigan were as much an aberration as Obama’s unexpected win in Indiana in ‘08; those two states will revert to form in 2020, just as Indiana did in 2012, leaving the above graphic as the probable map going into the next presidential election. Trump would need to run the table of competitive states just to achieve an electoral college tie and throw the election into the House (where, unfortunately, the Constitution's obscenely antidemocratic one-vote-per-state-delegation system would allow him to win 26-22 despite our new House majority, with the evenly split delegations in Pennsylvania and Michigan probably deadlocking and failing to cast ballots), while his Democratic opponent, whoever we nominate, only needs to win the blue states plus one gray state to secure the electoral college.
Moreover, I think this map also shows our path to holding the Senate in future Congresses after the 116th. Over the next decade, I think that the states in blue will almost always elect Democratic Senators to any open seats that occur in either a presidential or a midterm year, and in presidential years Republican incumbents will struggle to survive there as well (e.g. I expect that if we defeat Trump we’ll probably defeat Cory Gardner and Susan Collins along with him), while those in red will generally elect Republicans to open seats in both kinds of election (I don't see us holding the seats in WV or MT after Manchin and Tester retire), and often throw out Democratic incumbents when they have to run in a midterm year (witness the fates of Heitkamp, Donnelly, and McCaskill).
In addition, as demographics shift our way we should be able to win most open seats in the gray states that occur in a presidential year, and occasionally pick off an incumbent as well, while we’ll have a difficult, uphill battle to take open seats and may lose incumbents here and there in those states during the lower-turnout midterm elections. That gives us a “floor” of 44 seats and a “ceiling” of 60, for a median of 52 and a decent shot at holding the majority during the 118th Congress and beyond.
I think our strongest possible presidential nominee for 2020 would be Senator Sherrod Brown; he’s a progressive populist with very similar views to Bernie Sanders and Elizabeth Warren, yet he just comfortably won reelection to a third term from Ohio. He’s charismatic and has plenty of experience, but he’s a decade younger than Joe Biden and Bernie Sanders, five years younger than Hillary Clinton, and three years younger than Elizabeth Warren — not to mention six years younger than Trump. With him on the ticket, we should be able to nail down Ohio and Wisconsin, make Trump sweat over Iowa, and perhaps even make a play for Indiana, Kentucky, Missouri, and/or West Virginia, where the emptiness of Trump’s promises to revive the coal industry will be inescapably apparent by then.
We’d be putting his Senate seat in play, of course, but with Brown’s coattails, someone like Tim Ryan ought to be able to hold it for us. For his running mate, I would strongly suggest Beto O’Rourke — unless John Cornyn decides to retire, in which case I devoutly hope Beto will run for the open Senate seat in Texas.