I think UA's biggest challenge coming out of this is defending its hubs. Especially DEN, it will be in an uphill fight with WN who has all the money in the world to sustain a loss leading operation to gain market share. I would not be surprised if they buy out F9 to just get their gates at DEN. Remember, UA will also face WN pressure in the bay area, chicago and houston. UA's biggest challenge all across the country is a WN with a war chest of cash and very little debt.
When it comes to SE, I think FLL/MIA is not a great idea, but it will certainly be more open. AA I think will be a lot smaller at MIA given their weak financial position and slow return of deep south America flying. FLL will be quite open since both B6 and WN are likely to focus on bringing back other cities first. I don't think it makes sense for a legacy to build up at a primarily LCC airport. There will be plenty of space at MIA if they wanted to attempt it. I doubt they will. I don't see NK and F9 both remaining independent after this. Either they merge with each other or get taken over by WN or B6.
Does it really make sense for UA to buy an airline for narrowbody aircraft when used aircraft will be widely available at low cost from all the downsized airlines around the world?
UA, and before them CO, and WN have had a somewhat complimentary relationship.. I point to during the merger of UA and CO they choose to give the 18 EWR slots that belonged to pre merger UA to WN. Also they choose to lease their two gates at Dallas Love Field to WN. WN left IAH, and recently left EWR. They aren't really adversaries. Recently, before COVID, UA pulled back their CLE flying from DCA and LGA. The DCA slots UA decided to use to increase EWR-DCA frequencies. We never figured out what they were planning for the five LGA slots they were using for CLE. I would not be shocked if they discussed a lease to WN.
In Denver UA and WN seem to co-exist very well, UA stated it had become it's most profitable hub even in the face of WN's exponential growth. In terms of future growth at DEN I think UA made out like a bandit with the City of Denver adding new gates as well as acquiring a dozen or so on concourse A. In Houston there seemed to be a possible competitive face off as WN got the city of Houston to build a FIS at Hobby airport. UA and Jeff Smisek in particular made a huff. But in the end it didn't turn out to be much of anything, WN launched some new International routes, but ended up dropping most. The one's they still fly don't have much of an effect on UA's operation at IAH.
WN and UA co-exist in Chicago, Denver, Oakland/ San Francisco, Houston and Baltimore/Washington. I would look for WN to grow in LGA.
I think B6 probably has the most to fear from WN. WN has pretty much been pushing B6 out of LGB, they're the number 1 carrier at MCO and they handle twice as many passengers from LGA than B6. If AA were to sell some assets, such as slots in the NYC market, look for WN.