Oh, I didn't mean to imply that WN would start a NE Corridor shuttle operation. That's not their style--with the noted exception of HOU-DAL, of course

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I beg to differ. That style of service IS their bread and butter. Look at what they did intra-California after US deserted PSA's old routes. Their latest, MSP-MDW, is going right after NW, er DL, and UA.
I agree, 6 slots at LGA isn't much to work with... right now. However, look at things like BOS-ISP, BOS-PHL (wishful thinking, but wouldn't surprise me... might do as much damage as a shuttle route), BOS-BWI, BOS-IAD, and BOS-MDW and suddenly you make dents against the Shuttle. Look at what they've done in the SoCal area hitting not only LAX, but ONT, SNA, SAN and SJC. They hit the minor airports with shuttle type operations and leave the larger airports for longer haul routes/mini hubs (think BWI, LAS, LAX, PHX, BNA, MDW). It's too bad they don't fly into EWR (slot controlled?), HPN or Stewart, and they could potentially do some REAL shuttle damage out of BOS.
Here's another thought about BOS. What about some of the old bread and butter non-hub AL/US routes like BOS-BUF, BOS-ROC, BOS-PIT, etc? They could make them one stops to other places, MDW, BNA, DAL and HOU come to mind and make some interesting routings that business types might look at, especially with their fare structure. If done correctly, then WN not only goes after US but also UA, AA, DL and CO!
Now, with 6 slots at LGA, where would you place these in the WN system? My guess would be 3X BWI and 3X MDW. PHX and LAS would violate the LGA rules and you'd have to support connecting traffic out of those "hubs". Both BWI and MDW would give them enough O&D and has enough connections to make it work, until they could acquire more slots. Also, you get the bonus of going after US in BWI/DCA and UA/AA in ORD via MDW.
Speaking of slots, didn't ATA have slots at DCA?