I believe Delta would have little interest in F9. DAL's available capital is focused on building the new terminal at JFK and growing LGA with the inevitable slot swap deal. In addition, the prevailing logic seems to be the next deal for DAL is with AS when the moment is right. They would not want to be caught tied up in an F9 transaction if AS became available. After New York is settled, DAL knows it will eventually have to compete on the west coast...AS is by far the most preferred method to that objective. Finally, F9 is bleeding profusely. Their operation is worthless to DAL as they are besieged in their only hub by a powerful LCC (perceived LCC) in SWA and an emerging powerhouse network carrier in UAL. Buying them just for the narrow body A/C, would be like buying the Titanic just because you want the lifeboats.
DAL buying F9 is a crazy longshot. But, stranger things have happened...I think?