Currently Open-Minded gives the benefit "+3 Influence on Territories if Territories follow State Religion".  This is an easy way to free influence if you're winning the religion war within your territories, but doesn't really have anything to do with "open mindedness"; it's complacency your state's dominant religion.

Proposed Benefit:  +1 Influence on Territory per Religion present in territory.  In this way, the myriad of culture and coming together of faiths is what leads to a richness of culture. Your empire's Christians and Muslims and Jews and Hindus, etc., allowed to practice their faith, contribute to a richer culture of your empire.

I upvoted for the spirit of the idea but think it will end up being a poor choice, as religions tend to eat each others. You'll rarely get more than +1 or +2 (which is fine).
Maybe, there could be something added, that would help religious minorities to not get wiped out from those territories?

RedSirus wrote:

Yeah, I like the idea behind this (I mean, it's mine) but it would also need to come with some sort of shakeup that protects minorities and prevents snowballing.

It should be implemented a similar thing with cultures's zones of influence