From what I hear, the A321NEO isn't a true 757 replacement as some keep saying. Even though the fuel savings (and operating expense) of the NEO is supposed to be the selling point, and it is supposed to easily handle Trans-Atlantic, (albeit with less cargo), UA feels that the proven technology of the 757 is better right now. If the 321NEO can do ALL of the things that a 757 can, like short field performance; hot and high performance; and true Trans Atlantic range (even though we've been taking Westbound tech stops in the winter), UA will wait and see if all of the claims are true and will weight it. This sis not a knock on Airbus, it just that they can extend the 320 Family to match a great bird like the 757. I think that AA will use them on cross country flights in a premium config.
The 747-8 isn't a big seller for anyone (I think that LH is the only passenger operator?) IMHO, I think it would be a better seller as a freighter.
We will see what happens about the 777x. There has been talk that UA is interested in the 773, but it's just rumors right now.