-On domestic policy, I think he would spar a bit with the establishment, but let's not forget that (no matter your opinion of him) he's a very effective dealmaker. He would probably manage to get thins done, but in a way that makes him look like he's fighting the establishment.
-I imagine that Democratic gains would probably be mitigated by an unfavorable Senate map. If the GOP gains in the House in 2016 are as large as the map would suggest and they also hold their competitive seats, the Dems could have more inroads there.
-Whether he would get primaried depends on how popular he is: if his comments continue and make him unpopular, I could see the establishment fielding a challenger. However, a nasty primary would damage them badly going into the general, so I suspect they would just work to get him reelected.
-The 2020 field for Democrats would be quite large. I imagine that most people that opted out this year (Booker, Gillibrand, possibly Warren) would get in. If Castro is the VP pick this time, he could run. If Kamala Harris is elected to the Senate, she would also be an option. I think that, pending reelection, Tammy Baldwin would be an interesting choice. Andrew Cuomo could also go for it, but he doesn't seem like a good pick. I could also see Brian Schatz, Steve Bullock, or Chris Murphy running, depending on what they do during Trump's term.
The elites would never let Baldwin win. Many in her party wouldn't support her. she's completely unelectable
