I have no idea if this has been done before or if it can be done but I thought I would post it anyway. My first idea is for a religions unit. I know it has been done before but this one would be different. What it would be is that each religions would have it's own unit but they would all have the same stats. The biggest thing that they could do is that if they were attacking a city that didn't have the religion that they belong to they would get a bonus when attacking. also if they win the battle against the last unit in the city and take it then the city would then convert to their religions. This would do very well in game trying to win with a religions victory. Also it would make having different religions different.
My other idea is that a lot of the time when I try to kill a civ I take over they main land but they still live cause they have some small city hiding somewhere. I think to be more real that if when you take a city that if it has barrack that you get a copy of there map or maybe there plans to where all there units are. It could even be random where the larger the city the more likely you are to get something. Anyway I just wanted to throw out these 2 ideas. Feel free to say what you want.

Eh... Naw. Historically speaking cities' defenseive commanders were deliberately not given too many details about the defensive forces of other cities specifically for the reason that if every military installation contained more or less complete plans of the nation's defense, then if any city fell, they all would fall. Finding such documents that would give such clues would require intense investigation of more central archives(this would fall under the category of spies.).

Another thing, religious units aren't that bad an idea, but making them unique to particular religions may simply be too controversial. I'm thinking give each religion it's own crusader equivalent. It would require a Cathedral, Synagogue, Stupa, Mandir, Moque, Academy, or Pagoda in the city in order to build said unit, and it would exist in the place of the Knight or perhaps another unit depending on the religion.(Although it's requiremements and stats may vary, and said city could build still build knights. If one's religious civic is not Theocracy, said city will be able to build said unit for every religion that has it's Cathedral equivalent.) The Unit will function like a missionary, only it will have a combat value. Additionally, it will gain a 10% bonus for every religion that is in a city that it's attacking, and a 25% bonus if it's own religion is in that city, and it will have another ability called "repress religion". (The icon would look like the emblem of another religion that is in the occupied city with the slash through the circle over it.) When the ability is used, it selects a specific religion to repress. The unit will take damage as if it was attacked by a unit with a strength rating equal to the city's population with a "combat" promotion per building of that religion in the city and a +religion's influence rating% bonus. If the unit survives, the religion dissappears from the city, and all buildings of that religion are destroyed(including the holy shrine if it's there.). The city's population goes down by 1 to 2 if the attack fails, but if the attack suceeds the city's population goes down by 1+1 per happy person generated by all destroyed religious structures +1 if theres a monestary +2 if there's holy shrine.(-7 populants if the city has the temple, cathedral, monestary, and holy shrine for that religion) 1 unhappy person is generated per population loss, for 2 turns per populant lost. If using this ability causes the city's population to drop below 1 the city is destroyed. Destroyed religious structure generate pillaging bonuses comperable to towns, while destroyed holy shrines generate pillaging bonuses comperable to conquering cities. If the city is highly under the influence of the culture of a Civ with the Free Religion Civic, the virtual attack has a +10% bonus per religion in the city. Additionally under the Free Religion civic, the unit gains no bonuses for attacking cities with other religions in it, and the "repress religion" ability is unusable while under the free religion civic, making the unit essentially a kight that can act like a missionary. Using the "spread religion" ability damages the unit as if attacked by a unit with a strength value equal to the city's size, but it does not decrease the city's population, nor is sucess contingent on the unit surviving. If the unit dies or if the unit lives the probability of the religion spreading is half that of if a missionary had been used. Unit gains 25% bonus under Theocracy.

Potential names for said religious units, compared to their religions, and what unit they may replace.

Buhddism: Xiaolin Monk
Melee Unit
Replaces Maceman
8:strength: 2:move:
+50% vs melee units
+25% vs mounted units
+10% city attack per non-Buhddist religion in city
+25% city attack if city has Buhddism

Hinduism: Ksatria
Archery Unit
Replaces Longbowman
8:strength: 1:move:
2-3 first strikes
+25% vs melee units
+10% city attack per non-Hindu religion in city
+25% city attack if city has Hinduism

Judaism: Zealot
Melee Unit
Replaces Swordsman
8:strength: 1:move:
+25% City Attack
1-2 first strikes
+10% city attack per non-Jewish religion in city
+25% city attack if city has Judaism

Confucianism: Judge
Melee Unit
Replaces Maceman
9:strength: 1:move:
+50% vs melee units
+25% city defense
+10% city attack per non-Confucian religion in city
+25% city attack if city has Confucianism
Requires Iron or Copper

Christianity: Crusader
Mounted Unit
Replaces Knight
14:strength: 2:move:
Doesn't receive defensive bonuses
Immune to First Strikes
1-2 first strikes
+10% city attack per non-Christian religion in city
+25% city attack if city has Christanity
Requires Iron and Horses

Taoism: Calculator
Replaces Knight
12:strength: 2:move:
Doesn't receive defensive bonuses
Immune to First Strikes
Inflicts Collateral Damage on other units
+10% city attack per non-Taoist religion in city
+25% city attack if city has Taoism

Islam: Seracen
Replaces Knight
13:strength: 3:move:
Doesn't receive defensive bonuses
Immune to First Strikes
+50% vs Unique units of non-Islamic religion
+10% city attack per non-Islamic religion in city
+25% city attack if city has Islam

This is just a few ideas to compliment yours perhaps... If I get better at coding some of it might be done.

Yea I see what your saying about finding all the plans, but I think that if I took a city that some where in there I'm going to find the location to all the other cities of the nation you took it from. Or maybe just if I get your capital cause it is the headquarters of your nation and military

I like your ideas with the religious units. But I think it would work better if the were just a new unit and not replacing one because that would mess up some civs UU and also it could make some religions better then others. I also think that if the unit is attacking a city without it's religion then it would get a bonus then one with it. Because they would want to kill the "heathens" and convert them to their religion. I could also see it your way because the fact that they would want to free the people of the same religion.

As for the repressing different religions, The way I understood what you said didn't really look like it would be very profitable to do.

Well the idea isn't that you lose the ability to build the units they replace... One just needs the tech needed to produce said unit in order to produce the special unit(I.e. Iron Working, Civil Service, Feudalism, Guilds.) Replace isn't the exact word, so much that they exist in the same place as.

As for the repressing different religions, The way I understood what you said didn't really look like it would be very profitable to do.

It's not profitable to destroy religions. It pisses people off, and people die.

Ok I see what your saying. I sill think it would work best to have it be one religion unit that has the same stats. If not you have people thinking which religion has the best unit, I think it's best to leave it the way it was made so that all religions are the same the only thing that matters is if you have the same religion. But what ever, if your gonna make it into a MOD do what you want, I just wanted to say my idea and think what people thought of it.

My other idea is that a lot of the time when I try to kill a civ I take over they main land but they still live cause they have some small city hiding somewhere. I think to be more real that if when you take a city that if it has barrack that you get a copy of there map or maybe there plans to where all there units are. It could even be random where the larger the city the more likely you are to get something. Anyway I just wanted to throw out these 2 ideas. Feel free to say what you want.
Well quite frankly regardless of whether the city has a barracks, not being a warmonger myself, I rather like the idea, with the changes that it would not be the entire map of the opposing civ you get, but just a city's map (it's cultural borders - and btw, it would be nice to be able to trade city-territory-world maps) become visible to you. It would also be a percentage chance that you get anything based on the population of the city you take, with a chance that a really large city might give more than one city's maps to the invader...
so, you might get nothing on a city size 3 or less, but a city or three from size 4-9, and 50%+ with a city of size 10 or more, for example...

As for the religion unit converting a city...
it's too easy. Maybe if you restrict the unit to the use of someone with the Theocracy civic, but it's still a bit easy. That said, it would be nice to have the ability to remove a religion from a city, in the standard game, even if at great cost.

As for the religion unit converting a city...
it's too easy. Maybe if you restrict the unit to the use of someone with the Theocracy civic, but it's still a bit easy. That said, it would be nice to have the ability to remove a religion from a city, in the standard game, even if at great cost.

Yea it might be to easy that's why I think it might be best if it's like a 50% chance, also I think that the unit should not be very strong, like slightly weaker then the knight, but stronger if attacking a city without their religion.