Maybe they've considered it, but I doubt UA seriously considered BNA as a SE hub. There is plenty of land to the east of 2R/20L at BNA where they could put a whole new terminal with 70-100 gates. Nothing is stopping them if they want to take the plunge, but good luck.
You've pretty much identified one of the key problems with UA opening a hub at BNA. I don't think they'd need 70-100 gates, but they'd probably need at least 25-30 to offer a network competitive with WN, and there just isn't room to expand the current terminal to that extent. UA would have to commit hundreds of millions of dollars, if not a billion-plus, to get a new terminal built, and they'd have to be willing to spend at least a decade losing money in the Southeast against DL and AA (and WN). Don't forget that both DL and AA have relatively low airport costs at ATL & CLT.
Because the Southeast is a hole in United's network (just like TX is a hole in Delta's network). You literally can't reasonably fly United from the core Southeast to the core Southeast (Florida, Georgia, Alabama, South Carolina, Tennessee, most of North Carolina, etc - over a fifth of the US population). United doesn't even bother trying to compete on any of those routes because they can't do it economically. Seriously, look for intra-Southeast routes for United. Their flights are long and expensive.
You can't be everything to everyone. Competing with DL and AA for connecting traffic in the Southeast is a recipe for losing money for years. Yes, BNA could be a profitable hub decades down the road if business travel returns and robust growth in Nashville continues and air travel remains a viable mode of transportation and... I just don't see it happening.
What exactly did they do at SJC, BNA and AUS to make it a focus city? BNA never made sense since it is already a WN stronghold and SJC is literally right next door to SFO.
AUS makes sense, but they haven't added anything outside of hubs yet so idk what the strategy is there?
I think we just haven't yet seen a DL network expansion at AUS as part of the focus city strategy. DL had maybe 3 gates at AUS before the recent terminal expansion was completed in 2019? And then the pandemic hit in 2020. The first stage of the strategy would have been to connect AUS to all of the hubs apart from LGA; they didn't historically serve LAX, BOS, or SEA from AUS. The next step would be to add key O&D markets for local travelers.