Idaho and Wyoming

[Skill and Chance]

Wyoming is simply impossible due to how rural it is and how large the coal mining presence is.  They would have to completely flip on the environment and then some.

Idaho, on the other hand, could be a long run opportunity if Trump eventually causes Mormons to abandon the GOP for good.  They have switched sides very abruptly in the past, after all.  I also think there is some opportunity for GA-06/VA-10 style Dem growth in Boise and its suburbs.

[Skill and Chance]

WY will be R for the next 100 years and Idaho can trend D if there is growth in the Boise suburbs.

Why would super Republican suburbs growing help Democrats, bro?

"Why would super-Democratic counties growing help Republicans"- Texas Democrat in 1948

It takes time to actually flip states, but 2:1 Dem Millennials moving to the suburbs and exurbs en masse over the next 10-20 years would be the best thing that ever happened to the Democrats since the 1930's.  This is precisely how the GOP eventually got their version of the "demographic apocalypse" after 1968, by slowly but surely flipping suburban counties (and growing them by 3X while they were at it). 

[Skill and Chance]

Anyway the only way I could see this happening is if so many Democratic-leaning people move into Idaho and Wyoming that the composition of the state changes. The current population of those two states is never going to vote Democratic.

Yep.  People here never like this answer, but that is how most states change.  For example, right now young Virginians and Virginians who moved there from other states vote MUCH more Democratic than older and native Virginians, but it's apparently more appealing to focus solely on NOVA "switching parties" because the Republicans are "crazy" and stuff like that.  It's never the only explanation, but it always plays a big role.  The South quite simply got more Republican as more Northern conservatives moved there, period.  Vermont got more Democratic as more liberals moved in from NY/MA/CT/etc., period.  Colorado got more Democratic as more outsiders and Hispanics moved in, period.  The list goes on.

This is precisely why I think the conservatives cheering that Millennials are finally starting to move out into the suburbs are in for a rude awakening in the 2020's.