The speculation at the end of the video here is that it could be JetBlue or Spirit. I was thinking of VX myself, but I am not sure how much sense this would make (wouldn't they fly to bigger markets first, like Chicago, in the MidWest?).
I would really welcome an airline like VX at MSP, but it is a lot more likely that it will be Spirit. Although, with dtw as a hub for spirit and delta, I think that spirit will not last long. Also, with sun country offering low fares to leisure destinations I don't think it would work for spirit.