Because it's never too early, Senate rankings for 2020.
Safe Democrat: DE, IL, MA, NJ, NM, OR, RI
Likely Democrat: ME (Collins, the biggest limb I'm going out on here), MI (Peters), MI (Smith), VA (Warner)
Lean Democrat: CO (Gardner), NH (Shaheen)
Tossup: AZ (Special election, remainder of McCain's term, winner runs again in 2022), IA (Ernst), NC (Tillis)
Lean Republican: AL (Jones, he's probably got no chance but hey maybe they'll get Roy Moore to run again)
Likely Republican: KY (McConnell should rightly be destroyed, but is there a credible candidate? Amy McGrath may run for governor), SC (Graham), TX (Cornyn, Beto was a once in a generation candidate, the Democratic bench is now huge, but I just don't see it happening)
Safe Republican: AK, AR, GA, ID, KS, LA, MS, MT (unless Bullock runs for it), OK, SD, TN, WV, WY
So that's one GOP pickup and I see two very possible Democratic pickups (CO and ME). I think they can get Arizona and North Carolina, and I'll say they hold Iowa even though that state went pretty well for the Democrats this time (though they did lose the governor's race). So yeah, Democrats +3, which isn't enough to take the majority.
This will all change a million times between now and then, of course.