OK. This could be long, so I'll break it up into little posts. Pleas keep an open mind ;-)
In 2002 in New Zealand, a Nine-a-side Rugby game was staged between an All Black legends team led by former Manu Samoa halfback Tu Nu'uali'itia and a Kiwi legends team featuring former Kiwi/Tonga hooker Duane Mann. The game was a hybrid of Rugby League and Rugby Union. It started with four tackles/play the balls, guaranteeing the attacking team four possessions (if they held onto the ball for that long without turning it over through error), before switching over to rucks and mauls. The format reportedly worked quite well in allowing both teams to play on even terms, and even though the players had retired from professional Rugby, it was certainly the most successful cross code fixture as a contest (moreso than Wigan/Bath or Saints/Sale).
These are the additional rules I'd suggest:
Number of players nine players per starting team, 4 interchange players. Both codes have nine-a-side Rugby. Nines RL improved upon Sevens RL in that gameplay retained more elements of the structure and tactics of the full 13 man game, whereas it still offered the attractive element of the short game (e.g. SL World Nines). Similarly, nine-a-side Hybrid Rugby could better retain the structural elements of 15 RU than 7s.
Tackle/Phase Count 4 RL tackles/play-the-balls then rucks &/or mauls. This allows attacking team a set time in possession (RL) before allowing contest at every breakdown (RU).
Tackling charges using the point of the shoulder should be allowed (unlike in RU), as in RL, but use of the forearm in charging down kicks/playing at the kicker (as in RL) should be strictly penalised use of outstretched hands only. This would encourage and reward tough defence but still keep it clean.