By specifying WI, IL and IN and then "all states west of the Mississippi" I get the sense that the western states will be less a factor than the local ones? If a lot of your travel will be short hops around the upper midwest, then probably NW or another airline with good regional coverage will be the logical choice.

However, if your west-of-the-Rockies travel is going to be extensive, then I would look hard at Alaska Airlines, since (a) they are partners with all of the above, and EQM/P partners with DL and NW (i.e. if your midwest travels involve NW or DL metal you can get AS EQMs while you fly,) (b) AS has fairly liberal thresholds for their 2 elite levels, and (c) generally good redemption levels for award travel. Obviously, though, upgrades on non-AS metal will be an issue.

Good luck with your research.