My prediction for what will actually happen:
http://www.270towin.com/2018-senate-election/29JGB8Republicans end up with a 54-46 majority (gaining MO/IN/ND and losing NV). Obviously a lot of those are tossups though. I actually think McCaskill shouldn't be underestimated; I don't think her huge win in 2012 was purely because of Akin being terrible.
Regarding Baker and Hogan being SUPER-POPULAR:
1. Why would they even run? They have a pretty good shot at re-election as Governor, so why risk losing a national election? It would make more sense for Baker to run against Markey in 2020 (and he would also lose against him, but at least he has nothing to lose at that point).
2. The reason why they're popular is because they don't have to do much as Governor due to the huge majorities held by the Democrats in those states. In a Senate race, once voters realize what policies they would actually vote for in Washington, they'll vote to keep them home. Also, just because 74% of Marylanders approve of Hogan doesn't mean they're super-enthusiastic about him.
I mean, sure, if I were the GOP, I'd probably try to recruit Baker and Hogan for 2018. But don't expect them to win.