No I would not count of Horizon going any bigger any time soon. That would put them in the position of offering the same services on the same routes that Alaska does. And I think that is something that Alaska Air Group which owns both airliner would just as soon avoid.
Boeing may want as to purchase the 717 but I have heard that the move is to go to an all 737 fleet, apparently their borrowing a page from Herb Kellers book. That would leave them with the 737-200QC, 400, 700 and the 900. and really wouldn't be a bad deal for Boeing even though there would be a single 717