@The Great Dane is right, the worse thing the NRL could do is try to rapidly expand the event and run the risk of either increasing costs or splitting crowds.
Whilst it would be great to see the WCC or the USA v Canada game at Allegiant Stadium, remember the cost to hire the venue is around AUD1million a day, so adding a second day of matches is expensive. Putting the Test as a curtain raiser might work, and maybe Fox Sports US might pick up the feed and show on their secondary channels or OTT service, though I don't know if it's worth it for year 2, I'd suggest it would make more sense as a Year 3 or 4 aim.
If I was the NRL, I'd keep it as just a double header and be aiming to get it on sale as quickly as possible and see if with an 11 month build up if it's easier to move tickets. The last thing they want is to sell worse than this year, because we know the media will rip the concept to shreds just as quickly as they were to praise it's success this year.