Yx302, an insider contributor over on airliners.net, has posted some news he said was released today:
(1) Midwest has not been contacted by Boeing about returning the remaining 717's but they do not intend to keep them when the leases expire.
(2) E190's are planned to replace the 717's. Jetblue and Republic are willing to give up E190 order slots for them to acquire them, likely in the next year or two.
If this comes to pass, it would fit rampant speculation. But it's the first indication from the company that it is the plan. Yx302 had no additional information to share about timetables, financing, or anything like that.
If this comes to pass, Midwest's costs will seemingly be in excellent shape. Putting 100 people on a plane designed for 100 people is a big step. Keeping the in-house fleet to a single type (with variants) is another.
I would assume this would be the end of Signature, but that has become a smaller and smaller segment of their business. However as I pointed out on another board, even in 2004 almost 60% of the seats offered by the Midwest brand were standard coach seats, between the M80's and the FRJ/BE1 fleet. That's long before the CRJ, E170, and "Midwest Class" on the 717 came.