No way will it be Allegiant. Indy totally doesn't fit their model. Plus, they fly out of SBN, so if someone wanted to fly Allegiant it's not that far of a drive, just an annoying one.
I'm gonna also say no to Virgin America. Except for their attempt to go to ORD, this truly is flyover country for them.
I doubt it will be Spirit, since they are just getting their flights going again after the strike last week.
At one time, I believe British Airways used IND, but that ship has probably sailed.
I do, however, really miss Cape Air's service. I could really use the IND-SBN flights, would save me having to drive up 31 to get to Notre Dame games.