It seems to me that in setting up Florida only routes, F9 is setting up TTN for a merger. They now use TTN in a manner similar to the way that G4(?) uses ACY - leisure flights to Florida.
Push comes to shove, if they leave completely (which I doubt), I venture that Allegiant would come in. This would not clash with their ABE operation.
As for legacy carriers, the only one without a nearby hub is DL. I think a couple of E-170/175 of CRJ 700 flights daily to DTW (similar to what they do at SWF, which has a less populated catchment area, and also has B6) would be a logical next move.