If primaries are to be staggered, Illinois fits the bill for me - representative of the US ethnically, with a large city, suburbs, exurbs, rurals and a mixture of economic sectors and populations.
Illinois would be good, but Illinois is losing population. Look at census county maps and you see red---why not Georgia or Texas, or some other microcosm....
The second-best microcosm is Wisconsin, which would even had an advantage over Illinois, since it is one of the three bellwether states.
I don't think Wisconsin has any appreciable Hispanic or South Asian populations. Also between WOW and Dane exurbs it has some of the weirdest and most idiosyncratic suburbs in the country.
Wisconsin is a fascinating state to study politics because it's weird in so many ways. I don't think it's a good microcosm state.
Bronz (who I usually adore) is making a totally nonsensical point here (very strange and unexpected!).