From a personal perspective, 789 and ORD would be great. However the 772 and IAH are the more likely combination. Four 772 have just been modified or are about to modified to 240 EDTO which would help it do this route as close to the GC track as possible. Air NZ are on record that the current config in the 789 won't do North America well. I suspect that at least two of the aircraft arriving next year will be a lower density/more premium targeted at doing Vancouver daily. This may well free up the 772 capacity to do a fourth North America destination.
My two cents worth anyway....