I assume it will be difficult to get this sport off the ground legally with no governing body approval if they play 80 minute games of two halves, on a 100 metre field that has scrums and line outs with either 15 or 13 players on the field in any case.
Maybe 75 minute games, three 25 minute thirds, fields are 90 metres (soccer fields can be more in play with large in goal), 12 players on the field, 3 unlimited interchanges, 4 tackles with a roll the ball instead of using your foot, defence has to be only 5 metres back, last two players in tackle don't need to go 5 metres back. Any touch of the head is a penalty. 6 points for a hand passed score over the opponents line (i.e. a Try), 4 points if using a kick to cross the opponents line, just need to catch, not put the ball down (i.e. American Football Touchdown). Kicker chooses where to do conversations, 4 points for goal over 45 metres, 3 points for a conversion from side line or 40 metres out, 2 points if 35 metres out or mid range in from sideline, 1 point if front within 25 metres. Place or drop kick. Can be the same scoring system for a penalty and field goal. Use an NFL or AFL ball.
Interesting times...