I don't know what will happen with Trump in 2020, but I will say that Reagan lost 30+ seats in the House in 82 and then won 49 states in 84. Obviously Trump is NOT going to win 49 states but it sure didn't hurt his re-election chances to have Florida and Ohio elect Rep governors.
The biggest factor will be how the Dems choose to act now that they have some power. They have to walk a fine line between pandering to their base and trying not to tick off the moderates (or at least they should). For the first time in a decade, the Reps in the House can blame their broken promises on the Dems and not be lying about it so that could help get out Rep voters in 2020 as well.