There are important differences between RU 15s and 7s.
1st, place-kick is not allowed and PG/Conversion must be taken by drop-kick in 7s.
I think that this rule should be abolished and place-kick should be allowed in 7s. The reason for this rule might be place-kick takes a long-time. However, this problem can be solved by stopping time while place-kick is being prepared. Also, the same thing (stopping time while place-kick is being prepared) should be done in 15s.
2nd, the side of kick-off after scoring is different.
In 15s, the side who conceded points take kick-off. In 7s, this is reverse. The reason is explained that territory is more important in 15s, while ball possession is more important in 7s. This difference can be solved in this way: "The side who conceded points can choose which side takes kick-off." There will be no problem by this. There will be an occasion in which the side who conceded points choose kick-off by the opponent team in 15s. For example, in the case of turnaround in the last minute. The side who conceded points might consider that their kick-off would be catched by the opponent team and kicked out, and the game would be over. In this case, the side who conceded points might choose the opponent team's kick-off and try to possess the ball.
3rd, in 7s, a team cannot use all substitutes (only 3 of 5).
I think that this rule should be changed and using all 5 substitutes must be allowed in 7s.
There are other minor differences, but all important points can be solved in these ways.
And, about the time of game. Now the game time is 7mins half or 10mins half in 7s. I think that this should be changed to 8mins half. 8mins is good because that is 1/5 of 15s.