Let's begin with the map of which seats are vulnerable:
70% - Safe for Now
50% - Possibly Vulnerable (MN/WV/ME require retirements, CA requires the Top-Two Primary producing an R vs. R contest)
30% - Vulnerable
Gray - No Race
I rate Hillary Clinton as a 91% favorite to win her party's nomination and a 60% favorite to win the general, so assuming a Clinton presidency, here's an extremely early prediction:
ND: Cramer defeats Heitkamp 53-46
Republican GainMT: Zinke defeats Tester 49-47
Republican GainWI: Baldwin defeats Neumann 49-48
MO: Kinder defeats McCaskill 50-44
Republican GainIN: Ballard defeats Donnelly 54-43
Republican GainOH: DeWine defeats Brown 50-48
Republican GainWV: Manchin retires. McKinley defeats Goodwin 59-40
Republican GainVA: Kaine defeats Cantor 52-46
PA: Fitzpatrick defeats Casey 50-49
Republican GainFL: Nelson defeats Atwater 50-47
MI: Stabenow defeats Rogers 52-46
MN: Klobuchar defeats Seifert 53-42
NM: Heinrich defeats Sanchez 52-47
CA: Feinstein retires. Chiang defeats Swearingen 54-46
NV: Heller defeats Titus 50-46
AZ: Flake defeats Sinema 50-47
Republicans gain ND, MT, MO, IN, OH, WV, PA
Democrats gain nothing
Net: R+7