A. There isn't any room at BNA for a 30-40 gate complex, they could only expand either A or B (but not both) and both would net you maybe 15-20 gates max.
B. WN is too big now for UA to make a serious push and BNA isn't big enough of a market for both a UA hub and a large WN focus city
Ever hear of something called eminent domain? If UA decides that they really need a hub in the southeast and BNA is their choice then I highly doubt the city of Nashville is going to tell them to F off.
It's not just lack of space, it's the terminal building itself and the fact that it can't be very easily expanded. Other than an expansion of A or B, it is not very expandable. It is surrounded by roads/highways and terminals. You can't just eminent domain it. Rather than pursuing this "BNA Vision" project, BNA would have been better of bulldozing the terminal and replacing it with a more efficient design that could be expanded more easily (e.g. RDU/AUS).
Plus, you already have a large WN focus city that is now pusing 120+ flights, how is UA going to compete with that?
Alright, so let me start by saying that ANY of AA/DL/UA hubbing at BNA WILL NOT
happen. Nor should it, Nashville is not the type of market that will support a legacy hub.
But it actually is possible within the existing airport property to set up a (small) hub either with additions to the existing terminal (coupled with potentially moving some parking and ground utilities) or building a second terminal building.
Google Maps shot:
There is room for a small terminal (though nothing in the way of close in parking with it) in the northwest corner between the two runways. Alternately the Cargo areas could be replaced with a new terminal (that is, I believe, the site of the previous terminal building). Likewise, the ANG base could be replaced with a terminal. Either of those OR a new terminal could also be built to the east of the 3rd parallel runway where there is a lot of mostly vacant land...a good portion of which MNAA already owns or has plans to acquire should a 4th parallel runway ever be needed.
It will not happen and is not needed, but it is doable.
But UA will not be building a hub in the southeast. Not at CLT. Not at BNA. Certainly not at BHM or MEM. They don't have the resources to waste, they're not losing enough potential passenger traffic to make it worth it, and they have good enough coverage for people travelling to or from the area from somewhere else with their existing hubs.
Make something Idiot-proof, and the Universe will make a more inept idiot.