Which extremely red/blue state will become a new swing state?

[bidenwins!]

Tennessee is my guess, because I feel like the south will get more democratic.

[MargieCat]

I also voted for Tennessee.

It won't be a swing state until the 30s or 40s.

I voted for it because it is a fast growing state, but not to the point of Georgia, North Carolina, and even Texas.

[pbrower2a]

I'm going out on a limb on this one,  but I'm going to say Idaho because

(1) it has a small population
(2) it is attracting high-tech businesses that require people who generally lean D
(3) it is likely to get a huge influx of Mexican-Americans
(4) the importance of farming and ranching as a share of the economy is likely to decline.

This is the sort of state whose demographics can change rapidly. It seems reasonably pleasant and has plenty of high-quality recreation. Boise is about as warm in the winter as St. Louis, but without the  heavy rains and snows... remember that cold rains are even nastier than snow upon you You can brush off or shake off snow, but you can't brush off or shake off rain. Summers are cooler on the average than those in Grand Rapids, which isn't too bad -- except that it is a "dry heat", which makes it more pleasant. Extreme highs are close to those of Dallas, but... Dallas is a growing city or urban area, too, As in Dallas, "it's a dry heat". 

Double the population and Idaho becomes much like Colorado in the Double-Zero  Decade or Arizona now in politics.   

[Sol]

The kinds of people who Idaho attracts though are not exactly a Democratic constituency though. Northern Idaho in particular is in many respects a destination for people from liberal west coast cities, often as sort of a form of mega-white flight. IIRC there are tons of former LAPD cops in the Coeur D'Alene area, for example.

[𝕭𝖆𝖕𝖙𝖎𝖘𝖙𝖆 𝕸𝖎𝖓𝖔𝖑𝖆]

The kinds of people who Idaho attracts though are not exactly a Democratic constituency though. Northern Idaho in particular is in many respects a destination for people from liberal west coast cities, often as sort of a form of mega-white flight. IIRC there are tons of former LAPD cops in the Coeur D'Alene area, for example.

Yes, there are. In fact, Kootenai County (where Coeur d'Alene lies) has trended Republican in each of the last four presidential elections.