The same overconfidence that led Ds to lose a very winnable senate this year will come back around to Rs. I highly doubt Rs will have full control for 4 years, much less 6.
The only prediction we can make with any confidence right now is that Democrats will win one of the next several elections. Trump could well be reelected also.
I think 2018 will have encouraging results for Democrats but it may not be a mega wave. 2020 or 2022 have better potentials to be Democratic landslide years. Depending on circumstances of course.