Kansas, Iowa, New York, and New Mexico
I will include Vermont and New Hampshire since the question asked in relation to 2018 not 2020
Georgia can be a sleeper, if Kemp wins the primary (which he likely does) polling is pointing towards a margin of over 5 pts against Abrams. That wouldn't be as massive as the others on the list but a 4 or 5 pt swing in what is considered a deeply polarized state would be pretty impressive.
Isn't Scott retiring? I believe he is, and if so, the GOP will not come even remotely close to Scott's 15-point win in the state back in 2018, red wave (or even red tsunami, if there hypothetically was one) or no.
Also I can't see Kemp win by over 5 points.
To my knowledge Phill Scott hasn’t declared for re-election but hasn’t said he’ll retire.