Hi, I'm a lifelong RPG player, and I've just fought of something crucial to your game mechanics.
Like the RPG of Heros, and also Rolemaster Classical (1rst companion mainly), you should make a book about characters having advantages points for bonus, while having also disadvantages,, making ultra uniqueness for each persos ; extra bonus to stats, or attacks, spells, skills, playing perks, and the same for disadvantages like phobias, limitations, etc. A little like the sorcerer bloodline, but for ALL persos, get it?
It could also have like, destinies, god contact, special dreams, everything to further the deepness of everyone. It could be to have near persons that are to your charge (like aunt May in Spiderman!!) Or you could be the vassal of a powerfull entity (dragon, demon, or even celestial heaven entity like Deva!!!!!) It's infinite all the possibility (a little like the mermaid Azlan peoples and Aboleth!!)
It could all be selected by players, or by dice rolls......
By extending further this branch, each one persos'll fell even more heroics, unique, funny-playable and attaching!!!!!