Having a Republican in the White House is certainly an asset for a Democrat running for office in 2018. An unpopular one such as Donald Trump is probably going to exacerbate that situation. That may not be enough, for example to ensure Democrats a win in Connecticut next fall. Connecticut has had a series of issues during Malloy's tenure that has led to his personal approval sitting at 29% according to a Morning Consult poll from this spring. His disapproval sits at a staggering 64%. While Malloy isn't running for reelection, voter's opinion of him could color how the electorate views a Democrat running to replace him in 2018. This is the same scenario that helped elect John Bel Edwards Governor in Louisiana's 2015 race to replace Bobby Jindal.
Democrats also face challenges winning against popular incumbents in Clinton states such as Maryland, Massachusetts, New Hampshire and Vermont. Popular incumbents in those states are scaring off many top-tier challengers.
On the other end of the spectrum, Republicans could lose Governor ships in a slew of Trump states. Some of those states (Florida and Michigan) wouldn't be that surprising considering Democrats have done well on the recruiting end and the states were relatively quite close in 2016 as well as the last 2014 Gubernatorial elections. Other states, such as Kansas and Oklahoma could flip as a result of any combination of unpopular retiring Governors, hostile climate and good recruiting. In Oklahoma, Democrats have drawn an A+ candidate in 4 term Oklahoma State Attorney General and Vietnam War veteran Drew Edmondson. The outgoing Republican there sported a 42%-47% approval rating in the April Morning consult poll. Fallin's approval rating is still better than outgoing Kansas Governor Sam Brownback who holds a dismal 26%-65% approval spread.
Democrats could hold the Minnesota Governor's Mansion while losing the race in Wisconsin. Democrats have a wealth of candidates running (and even more that could still run) in Minnesota while the Minnesota Republican party has yet to recruit anyone above 2014 nominee and Hennepin County Commissioner, Jeff Johnson. In Wisconsin, Scott Walker has a 48-48 approval spread according to the latest MULaw poll, but Democrats have yet to recruit anyone to run. Though Tony Evers (one of only two state wide elected Democrats at the moment), who has expressed interest, would be a credible challenger.
I can't finish this post without mentioning: there is a homeless man running for Governor of Idaho, on the Democratic side, who started campaigning while in the county jail this spring. Democrats there are holding out hope that 2014 nominee, A.J. Balukoff, will run again. Balukoff is a businessman who ran a surprisingly strong race in 2014 against incumbent Governor Butch Otter in a very unfavorable year for Democrats.
A more practical guess at where the 2018 Gubernatorial races may end up would be something like this: