This maybe late, but wanted to input it in just in case.
Hope i'm not stepping in where I shouldn't but this does sound interesting and a bit complex. I'm from a era of were doing Role Playing Games like DnD on a BBS or a Group before Forums became popular, all we needed was a map, a character that was created, DM plan's some mobs, a couple mini boss's, a boss and has some ideas for the treasures of each character in the specific game and then our imagination flew from there.
This creating a plot is a bit of a twist for me. Here are some of my ideas, if this has already been discussed in the google docs among the group of you, then disregard.
Can always think about making a Hybrid role, were one can use a all types of methods. Spells of course as well as melee skills that will be similar to another role but with different stats attached to them to help balance out that character.
Melee - Have only two melee attacks that are the same as a Fighter role, 1 that is same as the Barbarian role, etc. But these would be your basic melee attacks. Until you decide which type of Hybrid you want to go down - Melee type or Magic Type. If you go down magic path then you have a 1 spell similar to a Cleric to help with healing and 1 that is like a Necromancer that can raise the dead, etc. (These of course are just examples)
Now if you want to add summoning like in FF of any version this is where they would be added, for the magic type. The choice really has to be made before a certain level or do it from the get go.
One of the melee roles could be a "Keyblader" that can equip only keyblade type weapons that can have bonus's attached to them cause of their specific role type. This will be more of a incentive to play a character from the game, Kingdom Hearts, cause this is where the role playing game stem's from; instead of making your own from another game or just from the get go.
There is also a path of going skill wise or stat wise.
Hope this helps with the discussing of ideas.