Here's something to think about. My comments are in brackets. - Wes
What if William Webb Ellis was tackled?
From a posting by Alison Brooks
At Rugby public school, a game of football [Americans: read "soccer"] was being played. One of the students, one William Webb Ellis, picked the ball up and ran forward with it, thus inventing the game of rugby football.
The myth is that he did something remarkable and innovative. In fact, there was no standardization about the laws of football at the time. The house rule at Rugby was that the ball could not be carried forwards, but it could be carried backwards, or propelled forwards. The Webb Ellis incident led to a "running" game that appealed to the thugs on the field.
But what if William Webb Ellis was brought down by a tackle within the first two steps?
It is fairly clear that there were, in principle, two basic types of football coalescing, the kicking game and the hacking game. The kicking game, which is now Association Football [known in the U.S. as "soccer"], would have been largely unaffected. The hacking game, which is now Rugby Football, would be affected. The rules appeared to have been moving towards the old Rugby Rules style, so the name Rugby might well still apply. If these had been the rules that did form, then carrying only in the reverse direction would have meant that the forward momentum of carrying would not be present. Tackling would be regarded as more of an attacking option (a successful tackle on a player means that you acquire the ball moving in the correct direction). Interceptions would also be more common.
Would the scrum develop? I believe this to be debatable. The ruck unquestionably would, as would the line-out. I'm less convinced over the scrum.
The knock-on does not arise under the proposed regime, which means that one of the limits to passing play is avoided.
A lot of parallels with Association Football would arise, and it is likely that Rugby would tend to be regarded as the aggressive form of Association (which would be limited to schoolboys, women athletes, and other wimps). The senior sport would be Rugby.