Interesting. I guess B6 wouldn't be the worst hookup. The problem is that the resulting airline ends up being a dumbbell: Good in NYC, good in Florida, good in California, the rest of the US is flyover country except for a tiny operation in DAL. That's not ideal (US didn't like being a PHX-PHL/CLT dumbbell either).
The problem with a DL hookup would be the Feds going "time to divest your slots"- after going to a great deal of trouble to snub DL at DAL/LGA/DCA they won't be letting DL walk in and take those slots in a merger.
(The same problem probably occurs with WN, UA and AA.)
I suppose AS could be a possibility (ALK has about 6x the market cap of VA, and actually is in better shape to handle a purchase than B6: more cash on hand), but the fleet and network dissimilarities would make it problematic, I think.