It seems clear, both going by the historical ways that midterms work and by the unique goings-on in this administration, that 2018 will be a solid Democratic victory, if not a "wave" (where a wave starts or ends is difficult to say). What'll happen in 2020 is harder to say, especially since both the Democratic and Republican primaries seem rather unpredictable at present.
I agree that there will probably be a solid Democratic victory in 2018, but I'm curious as to what your definition of a victory looks like.
I think Democrats will come close, but ultimately fail, to take the House in 2018. I think they'll prevent a Republican supermajority in the Senate, but still sustain a net loss in their numbers in that chamber. I believe their biggest gains will be seen in governors mansions and state houses across the country.
