I'd say best-case scenario for the GOP is the Democrats take about 30 seats in the House, maybe gain a seat in the Senate, and gain 500 combined state legislature seats and governorships.
I think the realistic scenario is a GOP loss of about 75-85 seats in the House, 3-4 seats in the Senate, most all the governor's races, and probably 750-800 state legislative seats.
Best-case Democratic scenario would be the GOP losing 100+ House seats, most all the senate races, and 1000 combined state legislative seats and governorships. And it's not inconceivable that it could happen that way given how the GOP has backed Trump and stirred up the Democratic base.
Based on primary turnout I haven't seen much enthusiasm by either party.