Not sure if it has been suggested, or even if it is possible, but a good idea is to add a complete color picker or just a box where we can enter our desired hex code. Sometimes the color we want aren't there in the defaults.
Balance all the characters so they have (almost) equal power then let the player decide what they want to use. If the player want to use the same four characters through the game, then let him, artificial restrictions to force the player to behave in a certain way feel bad.
It can make the player feel cheated, actually it can be complete BS if used incorrectly.
RNG should be used to decide equivalent outcomes or the outcomes should even out in the current scope.
In a battle, the foe has 1000 HP, your attacks deal 100 damage, if you have a 25%...
It basically just increases complexity as there is another resource to manage...
I like it as it allows more diversity in spells and adds more depth to the game.
For enemies: just give them a way to restore MP.
Standard if you have a lot of different characters to pick and form a party, you don't get individual character customization, but you have party customization. Also having to manage the stats of several characters would be pretty annoying.
Go with stats distribution if you have few characters...