Something to think about.
It is pretty clear that WN is moving to upgrade BNA to a base for the southeastern US; whether the base will be in addition to ATL or replace it remains to be seen. Furthermore, I think it's a pretty decent shot BNA could one day be on the same level as BWI and MDW, making it WN's third superstation in the eastern US. Since BNA is in a good centralized position in the eastern half of the US for connections, has a strong, growing local economy and generally has better weather and facilities (particularly the airfield) than those two (thus less delays) it makes sense to build up to that level.
The result of this may be WN needing additional gates beyond gaining all of C and D. Currently, taking all of C and D in their current forms will net WN 25 gates. The current master plan calls for an additional 3 gates to be built onto D, and since WN will take all of those, it will net them 28. However, longer term, they may want additional gates beyond that. For comparisons sake, MDW currently has 37 gates used by WN and BWI has 31, with an additional 5 coming in a few years. All in all, there is a possibility WN may need/want to take some B gates longer term as well. Following the A and D build outs, the current terminal will be maxed out and planning will begin for a second one.
So my thinking is, wouldn't it be better for AA and DL to move into the new A and build new clubs respectively there and make B for UA and the ULCC's, which then could be moved to a second terminal when the time comes, making room for WN to make a good chunk, if not all of B? I don't think it would be wise for DL to expand their Sky Club, only to have to vacate it eventually because WN wants more gates on B. Unless of course, the Sky Club will be accessible from the central terminal area post security and not directly attached to B.
I know I'm getting way ahead of myself here, but it is an interesting scenario.