I have a concept for crusades.
Launching a Crusade
Launching a crusade will be a new action that can be performed by an Apostle, when launching a crusade you must target one other civilization to be the target of the crusade, that civilization must be of a different majority religion than your dominant religion. Next you will also be placed in a special meeting of congress similar to the emergency system but only civilization following your religions will be invited. You can request that these civilization join the crusade, if they do so your religion will have increased influence in that civ's cities and that civ will join you in your crusade.
You may also only declare a crusade once every 50 turns (standard speed)
Crusaders
When an apostle launches a crusade, it will instantly retire and be replaced by a Crusader. Crusaders are a unique unit type that is trained with faith that are considered melee units. Crusaders have +2 more strength that that ages respective melee unit (so it will have +2 more attack than warrior in the ancient age, +2 more attack than swordsmen in classical age, etc...) and +1 movement when embarked. If a crusader takes a city that city will instantly covert to your religion and will give you 100 faith.
Adjustment to Arabians
The Arabians civilization ability The Last Prophet has an added ability that is this, taking a city in a crusade gives an additional 100 faith and Mamluks are included in the benefits given to Crusaders.
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I have returned, and hopefully will be more active. If you have comments or questions about this concept feel free to comment.