I don't know that UA will go for 777X - bigger than A350-1000 doesn't really fit their multi-hub route network or apparent aspirations.
Originally Posted by
Jorgen
A few routes you still need four engines, but UA doesn't fly any of those. (SYD-SCL or SYD-JNB though, for instance.)
Both of those are covered by existing ETOPS if all the diversion fields are available. The routing may not be wind optimal, but the flights aren't that long (~6000 nm).
Also Airbus is pushing ETOPS 420 which will provide more flexibility; I'd guess Boeing is looking at 420 too.