All Summoning Contracts are capable of having their own Sage Mode, though whether one is capable of learning this Sage Mode will be determined by a roll (Much like the success of Curse Mark Application). Variables can be put in place to increase the chance of success, such as lower ranks (D and C) requiring a 100 Dice Roll with a roll of say 1, 25, 50, 75 or 100 to be successful, while mid ranks (B and A) require a 10 Dice Roll with a 1, 5 or 10 to be successful, and finally the highest ranks (S to X) require a 5 Dice Roll with a 1, 3 or 5 to be successful.
There could also be other prerequisites after one succeeds their roll in order to learn Sage Mode. For example a training topic with their Contract's Elder that needs to be a certain number of posts to learn Imperfect Sage Mode and another topic with the Elder to learn Perfect Sage Mode.
When people make their Sage Modes, they can determine the boosts (following whatever limits were placed on the Toad, Slug and Snake Sage Modes) and the person is also capable of forming their Contract's own Kata, again following whatever limits were placed upon the Toad, Slug and Snake Kata.
I brought this idea forth because I understand the need to keep somethings within a small circle especially when many people have their eyes on it and it can be a wealth of power, having been a mod several times before myself, but I also felt as though the current system for Sage Mode is too restricting.
One a completely unrelated note: There should be a system in place that allows the gaining of more character slots. Because I know that I love drowning myself under the mountain of work that is juggling multiple characters. And I say that with absolutely no sarcasm. Having tons of characters is great.