It would be great to have a civ that is more religiously inclined than others. The vatican has been a dominant force throughout european history, and would be fun to play in the context of its struggle to hold onto religious influence and power in Europe. This is by no means a fully fledged design, but some cool ideas would be:
Villagers, and some (if not all) military units, and Religious Units cost faith, which is naturally generated through church placement.
Faith rate can be increased by placing of religious buildings across the map. These buildings naturally attract religious influence, can be placed wherever, and based off the number of villagers near them generate more/less religious influence.
Military and Colonial improvements are done through churches/religious buildings.
Leader is the Pope, whichever one is good for time period.
Units (suggestions that are not all based in any true history)
Missionaries (spread religious influence as well as have ok attack).
Crusaders (knights on horses, good against infantry)
Swiss Guard (strong knight on foot) https://www.telegraph.co.uk/travel/list ... iss-guard/
Could be interesting and has nice religious element with spreading religious influence across map that hasn't really be seen before in Age. However, could possibly be a little "too" different.