Well, it would be the same situation as the people who are fully maxed on their KG's. If a person plays long enough, they will fully max their KG slots regardless of whether it was a [Uchiha + Kaguya + Hyouton + Rinnegan] or the suggested Kyuubi 9 stacks. It's just a matter of time before everyone has "leftovers" provided they have the time, play the game, and are not a sitter.
My honest opinion though, is that the Kyuubi could either be turned into a select KG and treated as such, possibly with each new tail/reset requiring a higher Oro kill count, or that you can't just be an Uchiha and Kaguya and Hyouton and Rinnegan and have the curse seal/sst and have a biju as a mount. Personally I think it's a bit much. I also didn't want to implement this for leveling purposes, but more of a 1) more biju influence on gameplay, and 2) for a little more realism. It might even be good if the implementation makes leveling harder because it makes it hard to control chakra (like Naruto when learning various techniques, including walking on water). It would be interesting, fun, and challenging to have the biju be treated as a select KG or mount, which has it's downsides including but not limited to increased exp requirements for leveling, negative influence on chakra control/usage, side effects to hp and/or max health, and etc.
I like the idea of more mounts, or at least revision of them. Maybe amp up the Kyuubi/Shukaku but make it so you can only have it if you don't already have a KG or a particular KG. And I think that for being a very large portion of the Naruto world, the Biju should have at least a little more affect on game play and at least a little more a part of the game. Even the Kaguya which had only made a brief appearance in the anime is much more a part of the game currently than the 9 biju that most of the plot in Naruto is centered around currently.
Basically, I would like the biju to be more of an influence in the game than it is now. This is my opinion and a request. It is not up for me to decide how the game should run, or how to implement it, but this is what I think what make the game better and more interesting and fun.
Whether it is treated as a KG, limited, only achievable at a certain place/time, or an improvement of the current mount system, I would like to see more biju influence on game play. Even forest events would be a nice addition as well.
Again, feel free to agree or disagree. In the end, this is everyone else's game too.