Do you know what actually makes the games in the 90s look like they flow better?
The tendency for referees not to blow the play dead for a scrum after a knock forward. If the non-offending team gained possession 5m behind where it a scrum would be set, it would still be play on. As a result, you have games not being stopped as frequently.
Further to this, bobbles whilst playing the ball were allowed to be play on unless it was horrifically bad. Again, less stoppages.
If you want to speed up the play the ball, they don't need to bring in a six again rule. Remove the 'dominant' and 'surrender' tackle calls, and go back to simply that the player who effected the tackle must release the tackled player. Naturally the more dominant a tackle, the longer it probably will take to get up off a player. If they don't - blow the penalty. Allow the referees to follow through with it.
But of course, let's make it a coin toss for referees as to whether the same infringement is a penalty or restarting the set - because that makes total sense!