So we should start with the idea it'll be a Democratic year. Donald Trump is unpopular and he did poorly in highly educated suburban districts like GA-6 in 2016. These are the most vulnerable districts for 2018. Jon Osoff nearly took the district in the first election. It won't be a shocker if he wins it in this election. This is an open district. Open districts where the Republican is retiring are the most vulnerable. I'd be shocked if the Democrats don't win 80-90% of them. Right now FL-27 is the only other swing district we know will be open. If there are 5 such districts next year Democrats should win at least 4. If there are 10, they should win 8-9. The problem for Democrats is that there probably won't be more than 5 swing districts that are open. Right now KS-2 and OH-16 will be open. These are districts that both the Republican incumbent and Donald Trump won by at least 16 points. They are tough wins for Democrats even as open seats.
If Democrats get the majority they'll have to win a significant number of swing districts where the GOP incumbent is running for re-election. Those are harder to win, but parties do win them in a wave. Unfortunately, special elections are only held for open seats. So we won't get an idea how Republican incumbents will fare in November 2018.