Look at West Virginia or Kentucky over the last 10 or 20 years. As for where it could next happen, PA, WI, and MN are top contenders in my mind. ME could become competitive eventually as well.
What would make these places trend more right, exactly? None of them are like Kentucky or West Virginia.
I ing hate these types of responses, because I'm forced this TRUE albeit lame answer: SOMETHING will cause SOMEWHERE to trend right. You're being naive if you think every state is at least kind of on its way to becoming a blue state the "cooler" it becomes. Voters will react and react differently by the decade. Bush 41 won suburbanites by landslides in '88. Now they're a swing group. Rural Southerners were a swing group for a few cycles, now they're solidly Republican.
The current EC map won't last forever, and I'll be shocked if it lasts past 2028. Something big will happen to spark a change.