I wasn't going to post about it yet, but I've already been working on a Middle Ages mod for about two months now.
I started out fiddling with a straight-up scenario for Europe in the year 1100, but the more research I did the more I drifted towards making a general mod for the whole medieval period (500 AD - 1500 AD for my mod), complete with a new tech-tree, new wonders and many new buildings, new resources, new civics, new units and new victory conditions.
What I have done so far is mostly hours and hours of XML editing, research and design. I haven't done any graphics at all yet, mostly because I preferred to flesh out the design and programming first and also because for a while I couldn't be bothered to look for a program to work with DDS.
To be clear, what I'm working on is primarily a mod, intended to serve as a base for many scenarios in Medieval Europe. I intend it to be playable as a "normal" game, i.e. without loading any scenario.
A rough overview of what my mod features:
Period
500 AD - 1500 AD
Religions
Judaism
Catholic Christianity ('Catholicism')
Orthodox Christianity ('Orthodoxy')
Sunni Islam
Shi'a Islam
Resources
Copper, Iron, Silver, Horse, Sheep, Cow, Pig, Wheat, Fish, Clam, Crab, Whale, Dyes; work as in vanilla.
Spices, Silk, Incense, Ivory, Gems, and Gold; appear at the African and Asian edges of the map and represent resources available through distant trade routes. They are linked by building a Trade Route Improvement with a Caravan unit, which is consumed just like a vanilla Workboat.
New map resources
Salt, Olives, Amber, Rye, Camel, Beeswax
New resources available only through buildings
Cloth, Christian Relics
New techs
The mod has an entirely new tech tree so most of the techs are new; some examples are: Stirrups, Monastic Orders, Architecture, Classical Heritage, Crop Rotation, Renaissance Art, Astrolabe, Ocean Navigation.
Eras
Early Middle Ages
High Middle Ages
Late Middle Ages
New buildings
The list of buildings has not been finalized yet; most notably there will be many more religious buildings, particularly Catholic ones, such as Benedictine Monastery, Franciscan Convent, Dominican Convent, Templar House.
New national wonders
Not finalized, some examples:
Court
(replaces Palace as the default government center; Palace will be buildable as an "upgrade")
Royal Mint
Reliquary
(supplies Christian Relics, buildable with Catholic or Orthodox Saint (Great Prophet))
New world wonders
Again, the list is not finalized but a few I have so far:
Piece of the True Cross
(7 buildable, supply Christian Relics)
Dome of the Rock
The Umayyad Mosque
(Sunni Shrine)
The Imam Ali Mosque
(Shi'a Shrine)
Alhambra
Krak des Chevaliers
Malbork Castle
Mount Athos
New units
This is actually what I've least worked on. I mentioned that there will be a Caravan unit to establish trade routes. The early period should clearly be dominated by cavalry so the few infantry units available will be cheap and weak (apart from some Viking Axemen or something of the sort). The list will probably feature Cataphracts, some variety of Western knight types, Trebuchets, English Longbowmen, Scottish Highlanders, Viking Huscarls, Swiss Pikemen, some Eastern horse archer types, Galleys, Cogs. Needs more work, in any case.
New.... stuff
Yeah, this list is getting a bit long now. I'm almost certain I will rework the vanilla traits. Great People and Specialists will be different and somewhat more religion-specific. GPs names will be appropriate to the culture they're born to. Civics will be redone as well. Victory conditions will be changed, most notably I'm replacing the Spaceship with voyages of discovery.
I hope you'll be interested in joining my project!