All comes down to the way the ball is recycled in possession. Everything else is window-dressing.
If you are allowed to retain the ball in the tackle, then you can take more risks without fear of losing possession. You also have far fewer penalties and scrums because of breakdowns in play, as rucks and mauls are always going to be very prone to errors and infringements.
How to re-start - lineouts, competitive scrums, drop-outs etc - the scoring system, the number of players even, these are all issues which could be adapted. But as long as there is a requirement to release the ball in the tackle, and no secure possession, then you will have a game which is essentially about kicking for territory and hoping to score from a setpiece, peppered with lots of stoppages for scrums and penalties.
The decision on possession would essentially decide whether the game was league with amendments, or union with amendments. There is no middle ground.