As a fan of both rugby and American football I have been advocating for several years that if the popularity of rugby could be raised sufficiently then the USA team could be a major force in World rugby there are several small mountains for them to climb though before that will happen.
1. College football and the NFL command huge audiences and money and will prove difficult to get players to swap to rugby and potentially take huge pay cuts in the professional game. Many of the top SEC colleges do not have rugby teams Alabama, LSU to name two.
2. The few Americans I have spoken to who know rugby think rugby players are nuts, unpadded contact completely freaks them out and there is a video of training at Oklahoma college where a coach goes nots because a tackle is completed in American Football without pads becuase the coach was worried about injuries
All that said the quality they could produce is staggering potentially. A few players just drafted in the last couple of years from college backs this
Jadeveon Clowney DE 6'5" 19 stone and ran and 11.43 100m in 2011
Barkevious Mingo DE 6'4" 17 stone and change and 40yd dash 4.53s
Odell Beckham WR 6'0" 14stone and a 200m time of 22.31 s
And these are three of the top draft choices there are hundreds of athletes every year who do not make the NFL and there is no amateur American Football to speak of so the maths if you convert these athletes to rugby is scary.