The "Master" problem. and how many characters do you got mastered?

I mastered every plants besides some Chompers and some Cactuses.

For the zombies only the op variations of some classes.

Besides that i think that play a mastered character should give some reward.
People are scared about people will play only the same characters..
Well this seems more like a way to soften up balance problems for the devs..
People who play only a character or a class will not change anyway.. and those who want to stop grinding for a while can do it with some relax.
I invested more than 500+ hours for grinding exp and master every plant, but i dislike Chompers and Cactus. I would like to get some reward playing my favorite characters and turn back at training at classes i already mastered.
I'm not talking only for coins.. instead i find coins useless. i was thinking more of incredible ideas.. like:

- increase upgrades slot spending stars :Q_

- spending stars to unlock some unique customization after you continue play the class

- re-color?

Something that reward my love for character.
Nevermind if you love an op character, after electro brainz, ac perry and roses get nerfed i feel there is no really op character, but just strong or annoying characters (but still manageable)