I think Democrats turning WOW counties blue will be key to holding state. If rural Wisconsin starts voting like rural Illinois or rural Ohio, Democrats are toast in state without turning WOW counties blue. They are maxed out in Milwaukee and Dane counties. Washington County still very exurban so don't see it going blue anytime soon, but Dems should aim to get it to where Waukesha county is now. Waukesha county is a stretch, but if starts voting like McHenry county, Illinois (Bush in 2004 got similar numbers there to what Trump did in Waukesha county) it would be doable. Ozaukee County Democrats need to aim to flip outright.
Rural areas are going right and I don't see Democrats winning back the rural areas they've lost. They need to win traditional GOP suburbs to offset this. In many ways swing in WOW counties was not unusual. Pretty much everywhere in US with few exceptions, suburbs voted more Democrat than they normally do, even in the ones Trump held. By contrast most rural areas voted more GOP than normally do with notable exceptions being those near college towns and those where tourism main industry; they swung left, all others swung right.
Dems aren't maxed out in Dane just yet. I can see it voting 80%+ D in the future.
And its population is growing at a good clip, so even if Dems don’t improve their percentages further (which I think they probably will continue to do), the raw margins will still increase.