I like the idea, but I think having real bijuus in the game will give many people goals, and with goals motivation. Maybe adding something else into the game like ninja scrolls that villages can capture for temporary exp boosts for the villagers. Having Bijuu like this would leave Akatsuki without motive, and I'm sure other missing factions motives. There will always be a stronger player, its not hard to work with your allies and bring them down.