Rumor mill has it that either Milwaukee, or Madison. I'd actually lean more towards Milwaukee, as we have more Model S's here then Madison. Yes, Madison would cover the central part of the state, however, the numer of Tesla's drastically decreases out that direction. We have significantly more between Kenosha and Green bay then the rest of the state combined. And to top it off, we have Marshall Auto Body, the only Tesla body shop in Wisconsin, so it only makes sense to have everything in one place.
It has been discussed within Tesla that they really need a service center in Wisconsin, and Maybe a gallery.
The anti-direct sales law, BTW, has been on the books since, I believe it was 1933.