If you had to guess what Biden state will be a national bellwether...

[Non Swing Voter]

I'd go with Pennsylvania.  It has a lot of elements of a lot of states the same way Ohio used to.  The country is getting more and more urban so I think you need to pick a state with a fairly large metro area.  Philly seems to fit this better than say Detroit or Milwaukee.  It is also a state with a lot of rural and suburban elements as well as smaller cities and exurbs.  It's also not as polarized along racial lines as say Georgia.  The demographics are a bit whiter than the country as a whole but it certainly encapsulates national elements of growth among hispanics and latinos, particularly in the Philly suburbs. 

The answer to this is also heavily dependent on if Texas flips and becomes a reliably democratic state.  That changes the math considerably and then a state that is currently considered solid blue like a Colorado, Oregon or Virginia would be the bellwether.