One key factor is the Green Bay Packers. A new format would have to have some compatibility with PBP. And yes, the games would have to be on FM. Granted, the games could go to WRIT or WMIL, but I don't think IHeart would want to disrupt them (they had issues with UW sports on WRIT before starting up The Game).
And again, WISN is not going to get an FM simulcast. First, they do well enough on AM, they don't really need an FM signal. Second, it's an aging format that isn't widely attractive to sponsors. Third, the speculation is a music format, not talk. And then there's the issue of whether they can get permission from Hearst (owner of WISN-TV and the WISN call letters) to allow the use of the call letters on an FM station, if IHM desires that. So, WISN-FM is not gonna happen.
So, I'll stick with my original guess of Alt. Rock. And it will probably be a voicetracked mess with very little local presence, like their other Milwaukee music stations. Local presence is what really helped WLUM for years.
If any station was going to do alt, I figured it would be WRXS (106.9). Saga would most likely do a better job.