Holy Wars

I think they should institute some sort of Holy War system. With the ease of spreading religion in BNW, it would be a great system to add.

Civ 1 is Religion A
Civ 2 is Religion B

Civ 3 is Religion B

Civ 1 attacks Civ 2, no issue.
Civ 1 attacks Civ 2's Holy City, Civ 3 declares war on Civ 1.

Also, can add a little bonus incentive that Civ 3 gets a 100% faith bonus while fighting the holy war.

It would be a fun way to create some more war and havoc and also giving Holy Cities more importance.

Religion is a little bit off. I suggested elsewhere that there be benefits for using missionaries and inquisitors even if you haven't founded that religion, giving other players incentive to spread someone else's religion.

I think a Holy War could function by way of simply making it to where someone else capturing a Holy City upends the religion, weakening the benefits followers can get. If you have some really sweet Missionary and Inquisitor beliefs, then you really don't want that to happen, and thusly you have an incentive to declare war to either liberate or take the religion for your own.

That's another thing, I do think you should maybe get founder benefits if you capture a Holy City. That, in and of itself, might single-handedly solve the current BNW problem of nobody having motivation to wage war, and also religion feeling irrelevant. If you could get all those delicious benefits of Founder Beliefs and control over two religions, that would definitely spark interest in launching a campaign against your enemy. Or, possibly, you could instead choose to eradicate the other religion, strengthening your pressure in religiously competitive zones and providing you more benefits.

As it stands, capturing an enemy's capital is often more of a burden than anything, since you can get way more bonuses from trading with them. If you could get the Founder belief a Holy City has, then you would definitely get some nice reasons to keep that capital around.

So how about making it so that
Capturing a Holy City gives you full control over a religion. This gives you a few options:
1) You can integrate its founder beliefs into your own religion and slowly eradicate it over x turns.
a) During that time, other Civs who follow the religion have increased incentive to attack you to take back the holy land (holy war). If the other Civs who follow the religion manage to take back the holy land, then they gain full control of that religion.
b) Once the amount of turns ends, the holy city can no longer create great prophets for that religion and the city no longer follows the religion.

2) Tolerance: You allow the religion to coexist with your own, but do not gain founder benefits.
a) Holy wars cannot be initiated in this instance.

One of my favorite things to do is get the Just War religion enhancer. It's great for when you have an AI enemy that doesn't have a religion yet. You can just send missionaries/GProphets to the cities you're targeting and bam! You get a +20% combat near those cities. Also works on CS, too.

In a recent game, Poland was going hardline religious and when I converted his capital with a prophet, he declared war. I'm fairly sure that was his main reasoning behind it, as the religious modifier was by far the most bright red. So I'd say holy wars do exist, after a fashion.

However it's worth pointing out that beyond the founder of the religion, the holy city itself has zero value. You can't even get a bonus from capturing it, even if its of your majority religion. It's a shame really; I'd like to see at least some bonus for capturing a holy city.