Here's the thing.
The Democratic Party has been focused on presidential years, not midterm years, of late. (Observe that they made mild gains in Congress in 2012 and 2016, and took huge hits in 2010 and 2014.) So already the Republicans have this edge on them in 2018 unless they can seriously reverse course.
Thanks to that disastrous showing in 2010, the House is gerrymandered all to hell against the Democrats. I can't really see them making up 30 seats they'd need for a majority there.
Nearly half of their Senate caucus is up for re-election in 2018, who came in on the back of Obama's 2012 win and the one midterm election of late where the Dems did well, in 2006 (because Dean actually had a fifty-state strategy, unlike Wasserman Schultz who was just a good fundraiser and had to raise tons of money to pay off the campaign debt Obama dumped on the DNC). When the 2008 group, who also rode Obama's coattails, came up in 2014, the Dems got hammered. 10 of the Democratic Senators up for re-election are from states Trump won, and not only that, but in the meantime the Voting Rights Act has been emasculated and Republicans, who control many of those states, have enacted tough voter suppression laws. So I can't see them picking up another three Senate seats--in fact, I think it's more likely they'd retake the House than the Senate, and I think the Republicans will actually extend their Senate majority.
So the VRA won't get updated, the voter suppression laws won't get struck down, the House maps will largely remain in effect for 2020, and it'll take something really, really special for the Dems to make big gains there. They could well beat Trumpelthinskin, or the religious fundamentalist who would replace him if he were impeached, but they've dug themselves a deep, deep hole and it will take a big change in the party to get themselves out of it any time soon.