Don't get me wrong, I do indeed think that NW going with the F-100's is very unlikely. Not totally impossible, should they decide to do it, but not a likely path.
Maybe NW will just start picking up more (newer) DC-9's. Although if they were going to do that I would imagine they would have gone after the -32's that US retired (all of which I believe now have flown off to new owners).
Here's what I see as possibilities, ranked in likelyhood of happening:
1) Nothing. The oldest DC-9's are retired. NW has already said they were going to slowly reduce the "DC-9 role" to 100 aircraft, so they can afford to retire some 55 aircraft before having to worry about it, and then they could...
2) Buy some lower-time DC-9's from other airlines to fill the gap
3) Go with deeper fleet rationalization and operate the A318. Perhaps for a 100+ aircraft order Airbus will build a A318 "short range" which would help close some of the operating cost gap.
4) Purchase the 717
5) Purchase the EMB-190 or other "uber-RJ" stratch machine.
6) Purchase MD-80's from CO or DL, which are looking to go all-Boeing, and really all 737 in narrowbody. Sure, they are bigger than what NW needs, but they should have a decent balance of purchase cost and operating costs.
7) Purchase the F-100's