I suspected that if and when UA decided to add flights to PEK, it was going to be LAX-PEK as UA has repeatedly stated no non-TYO flights except for LAX/SFO with the lack of Russian airspace access. I reiterate that it is not because of the range of the 789s rather the extra flying pushes flights from making money to breaking even to losing money on almost every flight. The second LAX-HKG flight really is a replacement for EWR-HKG as that flight allows for onward connections at almost every domestic station UA serves at LAX whereas the initial flight was mostly catered to LAX basin passengers with minimal, but not zero, connecting flights. Only surprise here is that this is only thrice weekly instead of five times weekly as that would fill up the current slot allocations to US-PRC flying for US carriers. (This includes Delta resuming LAX-PVG thrice weekly in June.) The pandemic and its aftermath plus the closure of Russian airspace to US carriers has allowed UA to reestablish itself at LAX with longhaul flying while AA just gave up on the non-JV longhaul flying and no intention to return and Delta has been rather slow to return to it, while giving up on LAX-LHR. To be fair, UA is only going daily next NS on LAX-LHR and AA is also pulled back a flight, signally there is way too much capacity, especially in the back, on West Coast-LHR flying.

I will go out on a limb and say the next longhaul LAX flight to start/resume will be LAX-SIN within the next two years as UA brings in the IGW 789 with a strongly rumoured more premium configuration. Primary reason LAX-SIN only lasted a year is UA discovered that they had to block seats in the rear on flights during the summer, unlike SFO-SIN where a few seats are blocked only on the worst winter days, making the route just too far for the 789 but the extra range with the IGW version coming out shortly plus a potentially significantly less dense version of the aircraft make it viable again.

Only other route I can think of UA starting up is LAX-ICN if and only if the Korean/Asiana merger is approved and they are required to curtail their combined LAX-ICN flying, which is flown four times daily, all on A380 aircraft, and the reduced flying leads to an opening where UA is able to soak up enough Star Alliance members on both ends that a nonstop flight with a 789 can be sustained. Big reason why Asiana and Korean have managed to make LAX-ICN work with double daily A380s respectively is connections at ICN onward to Asia. UA obviously won't have that but would have connections at LAX, depending on the flight timing.