There really isn't any airline that AA, UA or DAL want or need in the US. No airline in the US would offer them anything they need. We saw this with DL in SEA, they could have bought AS but decided it would just be easier to move in and build their own hub. Same thing with UA and Frontier or AA and Spirit.
The fact is the big three have all they want and need in the continental US. There isn't anything any airline offer they couldn't just buy or build on their own.