Mainly everyone in the game is going to get +10 nin/str once the RB skills are finished. Even now you see this with the Iron RB passive. Those are obviously the favored choices, but to keep them from being the dominate RB passives, I think a perk for focusing in certain stats would help balance it out.
These are the passive you would get 40 of these stats.
Speed: Either Rank 0 run speed. It already gives some kind of slow resistance right? That should even it out with the others.
Stamina: +15% damage defense.
Handseals: -10% cooldowns
Intelligence: Do .05% more damage for every amount of current chakra you have.
Taijutsu: (may have to add a RB passive that gives +10 tai) Has a 20% chance to attack the opponent twice with every F key attack.
Str/Nin wouldn't get a benefit, since they give great benefits already.