It's a dilemma. On the one hand, you want the US and Canada to have success. You want to give both sets of fans a great spectacle to see. In my mind, that would be a USA vs Canada quarter final.
But its also important to have balanced pools. Moving away from super pools.
Using the '17 nations plus Canada and Russia, here is how I would have the pools.
Pool A; Australia, Scotland, Fiji, Italy.
Pool B; New Zealand, Lebanon, Tonga, France.
Pool C; England, USA, Russia, Samoa.
Pool D; Canada, Wales, Ireland, PNG.
Going off that, the likely/ideal quarter final scenarios would be ;
1st A (Aus) vs 2nd B (France)
1st B (NZ) vs 2nd A (Fiji)
1st C (Eng) vs 2nd D (Ireland)
1st D (Canada) vs 2nd C (USA)
I would consider staging a double header in Hawaii. USA vs Samoa and NZ vs Tonga.
This is all in an ideal world where development into both national teams is strong and they get to a level where they can be reasonably be expected to compete and beat the likes of PNG, Ireland and Wales. I doubt it though.