In Civilization 4, when I play a game on any setting(beside Emperor and Diety and all that batcrazy stuff), half of the world are under my religion, and half of the other world are under something else. Wouldn't it be fun, if we had Crusades? Crusade would affect everything.
How a Crusade would start.
Let's say you and Isabella shared a religion together, otherwise a Crusade wouldn't start without two civilization of the same religion.
Isabella: Care to purge the world of heresy?
Me: No, we respect other's religious beliefs.
Isabella: -10 You decided to ignore us on our effort to cleanse the world of heretics.
Isabella declares war on you for having a civilization full of heresy and finds another person to start another Crusade with. The end.
Isabella: Care to purge the world of heresy?
Me: Yes, heretical scums must die! They must convert or prepare to suffer the wraith of [insert religion name]
Isabella: +10 Our brothers and sisters of the faith has come alongside to help fight against the evil.
After the Crusade has started, everybody in your civilization and Isabella's civilization goes anarchy for 1 turn regardless of traits. After the anarchy disappears, you and Isabella has declared war on everyone who decided not to help you or her with the Crusade efforts regardless of same state religion. Everyone must be involved, they can ask for peace by converting to the state religion of the Crusade. Otherwise, they must forge a Crusade of their own to fight against this war.
Every city will have happiness or unhappiness for...
This Crusade will help us achieve that no one messes with the [insert religion followers]
We wiped out a heretical civilization.
Our brother and sisters of the faith will leads us to victory.
We razed a heretical city.
We have taken back our brother and sisters of faith. (Capturing a city of your state religion)
We are glad our city is not affected by heresy.
Slavery is helping us punish the heretics through their labor. (For using a whip in a captured heretical city.)
We are glad our government is guided through religion. (Having Organized Religion or Theocracy as your civic)
We are happy by having our city be in a police state to prohibit heretics and heresy.
We are glad that for having a King that was chosen by god. (Having Hereditary Rule as your civic)
We are glad our people are strong enough to defend itself. (Having Nationhood as your civic)
We are happy that an inquisitor have came here to destroy all of this heresy in our city.
x 10 We are happy that an artist has decided to influence our people on the danger of heresy.
We razed a heretical holy city.
The Crusade is a hopeless war! We should give up! We're losing more what we can get back!
We lost one of our brother and sister of the faith! (Losing a civilization of your state religion.)
Our holy city was lost to you through your useless strategy!
Our holy city was razed!
Our city was destroyed by the heretics!
You decided to capture a heretical city? (This is ignored, if you have slavery civic)
You decided to capture a heretical holy city?!
Our city is full of heresy! (Different religions in your city, but is ignored if you have recently whipped.
WE DO NOT WANT HERETICS TO BE SPARED. (Having pacifism as your civic)
WE DO NOT WANT YOU TO ENCOURAGE HERESY. (Having Free Religion as your civic)
YOU'RE ENCOURAGING HERESY SPEECH. (Having Free Speech as your civic)
Emancipation is useless for us, when you think that heretics should be EQUAL! (Having Emancipation as your civic)
Everytime, a city produce a unit, that unit will receive 50% more xp. For example, if I had recruited a Warrior, he would have 4 xp because of his Vassalage and Theocracy civic, but instead he's getting 6 xp through the 50% because he is trained in a Crusade to fight for a better cause.
In a Crusade, units have access to 3 Crusade-only promotions which is Warpath I, Warpath II, and Warpath III. This means these promotions an only be active when a Crusade is active. Warpath allows units to have +1 movement range. Warpath II allos units to have +1 movement range. Warpath III allows units to have +1 movement range.
Furthermore, in a Crusade, Great People except Great Spy and Great Scientists plays a great role. Let's say a Great Prophet, a Great Prophets can build a holy shrine or.. get you +300 gold for each religion removed to remove all of the religion in a heretical city or.. can lead units by giving them 3 xp EACH. Great Artists can be a culture bomb or.. a portable happiness meter by allowing that the city that the Great Artist has settled, 10, which will disappear after the Crusade has ended. Great Engineers can rush units, buildings or wonders, or they can give a city 50% city defense to all units that has fortified in it. Great Merchant can give you ton of money on trade mission or.. give you +100 gold each on a holy city for each city that has that holy city's religion. Great General had already played a role, so we won't explain that.
After the Crusade has ended, when you decided to quit the Crusade causing your Crusader allies to be mad, or when you conquer all of the world leaving only your Crusader allies...
Well, right after the Crusade ended... everything goes back to normal. Yay..
How a Crusade would start.
Let's say you and Isabella shared a religion together, otherwise a Crusade wouldn't start without two civilization of the same religion.
Isabella: Care to purge the world of heresy?
Me: No, we respect other's religious beliefs.
Isabella: -10 You decided to ignore us on our effort to cleanse the world of heretics.
Isabella declares war on you for having a civilization full of heresy and finds another person to start another Crusade with. The end.
Isabella: Care to purge the world of heresy?
Me: Yes, heretical scums must die! They must convert or prepare to suffer the wraith of [insert religion name]
Isabella: +10 Our brothers and sisters of the faith has come alongside to help fight against the evil.
After the Crusade has started, everybody in your civilization and Isabella's civilization goes anarchy for 1 turn regardless of traits. After the anarchy disappears, you and Isabella has declared war on everyone who decided not to help you or her with the Crusade efforts regardless of same state religion. Everyone must be involved, they can ask for peace by converting to the state religion of the Crusade. Otherwise, they must forge a Crusade of their own to fight against this war.
Every city will have happiness or unhappiness for...
This Crusade will help us achieve that no one messes with the [insert religion followers] We wiped out a heretical civilization. Our brother and sisters of the faith will leads us to victory. We razed a heretical city. We have taken back our brother and sisters of faith. (Capturing a city of your state religion) We are glad our city is not affected by heresy. Slavery is helping us punish the heretics through their labor. (For using a whip in a captured heretical city.) We are glad our government is guided through religion. (Having Organized Religion or Theocracy as your civic) We are happy by having our city be in a police state to prohibit heretics and heresy. We are glad that for having a King that was chosen by god. (Having Hereditary Rule as your civic) We are glad our people are strong enough to defend itself. (Having Nationhood as your civic) We are happy that an inquisitor have came here to destroy all of this heresy in our city. x 10 We are happy that an artist has decided to influence our people on the danger of heresy. We razed a heretical holy city. The Crusade is a hopeless war! We should give up! We're losing more what we can get back! We lost one of our brother and sister of the faith! (Losing a civilization of your state religion.) Our holy city was lost to you through your useless strategy! Our holy city was razed! Our city was destroyed by the heretics! You decided to capture a heretical city? (This is ignored, if you have slavery civic) You decided to capture a heretical holy city?! Our city is full of heresy! (Different religions in your city, but is ignored if you have recently whipped. WE DO NOT WANT HERETICS TO BE SPARED. (Having pacifism as your civic) WE DO NOT WANT YOU TO ENCOURAGE HERESY. (Having Free Religion as your civic) YOU'RE ENCOURAGING HERESY SPEECH. (Having Free Speech as your civic) Emancipation is useless for us, when you think that heretics should be EQUAL! (Having Emancipation as your civic)Everytime, a city produce a unit, that unit will receive 50% more xp. For example, if I had recruited a Warrior, he would have 4 xp because of his Vassalage and Theocracy civic, but instead he's getting 6 xp through the 50% because he is trained in a Crusade to fight for a better cause.
In a Crusade, units have access to 3 Crusade-only promotions which is Warpath I, Warpath II, and Warpath III. This means these promotions an only be active when a Crusade is active. Warpath allows units to have +1 movement range. Warpath II allos units to have +1 movement range. Warpath III allows units to have +1 movement range.
Furthermore, in a Crusade, Great People except Great Spy and Great Scientists plays a great role. Let's say a Great Prophet, a Great Prophets can build a holy shrine or.. get you +300 gold for each religion removed to remove all of the religion in a heretical city or.. can lead units by giving them 3 xp EACH. Great Artists can be a culture bomb or.. a portable happiness meter by allowing that the city that the Great Artist has settled, 10
, which will disappear after the Crusade has ended. Great Engineers can rush units, buildings or wonders, or they can give a city 50% city defense to all units that has fortified in it. Great Merchant can give you ton of money on trade mission or.. give you +100 gold each on a holy city for each city that has that holy city's religion. Great General had already played a role, so we won't explain that.After the Crusade has ended, when you decided to quit the Crusade causing your Crusader allies to be mad, or when you conquer all of the world leaving only your Crusader allies...
Well, right after the Crusade ended... everything goes back to normal. Yay..
Crusade might be an event when You're running state religion and perhaps a quest to get some infidel Civ under Your heel as a vassal ? That surely would be fun but I doubt that AI could compute all that choices You've proposed 
That would be one hell of a script ! hehe 
