Frerichs putting himself out of the Il-13 race leaves Dr. David Gill who has expressed an interest. Problem is that he is already a 3 time loser having lost to Johnson in 2004, 2006, and 2010. However, the district is quite a bit more Democrat friendly than the present Il-15, but if the Dems can not do better than a 4th time retread, then I expect Johnson to be reelected handily.
Also, Frerichs' increased margin in 2010 had more to do with the looniness of the GOP's candidate (tossing a great pickup opportunity out the window) than anything Frerichs did or did not do. I believe the GOP organization refused to even endorse their own candidate in that race.
Ill Ind
Mike Kelleher could beat Johnson (though it would be very close, I could see it being more or less a reverse of the 2000 election).