LAX-HKG seems like a very safe bet for a potential route. While the 789 sounds good in economic theory (reduced CASM cost), the cargo demands and demand for premium seats on that route ultimately points to a 77W hauling the mail.
The thing that I feel like most of us pontificators are forgetting is that there is a limited # of 77W's that AA has flying around. With the SYD route, an additional LAX-LHR frequency mentioned upthread, and other moves, there can't be that many 77W's that are unspoken for, no?
Are there routes the 77W is currently running that a 789 would be better suited to perform? South America comes to mind.