The Democrats will take control of the Senate. (And their first action should be to change the filibuster rules. Do it immediately while the iron is hot, or they’ll regret it later.)
The House is in play. This is where the risks of gerrymandering will become apparent. When districts are arranged so that one party has a slight advantage in many, they’ll do well in typical years. But when there’s a tide change, that slight advantage can drain away in every district at once. I expect some states to have massive shifts in their representation (Ohio, for example).
The biggest risk Democrats face is low turn out. Everyone will see that they’re going to win, so why bother to vote? Because of this, all the players (Democrats, Republicans, the media) are going to make the race look tighter than it really is. We saw this to some extent in 2012. It’s going to be more of the same this year. Enjoy the ride!