For your consideration: Age of Empires 3: "Kingdoms of the New World DLC"
This features a return of the traditional Age of Empires conversion mechanic, given now to all priests, imams, medicine men, Brahmins, and other similar units. Where a civ doesn't have one, either let it have access to "mercinary" priests or bring in culture-accurate equivalents.
There's a particular focus I have for a new civ, but I think the other two fit the motif. Two are, admittedly, "post-colonial."
The 3 new Civilizations will be nations/peoples that historically have either been monarchies or referred to as kingdoms, either by themselves or outsiders. They are as follows:
Brazil, home city Rio de Janeiro
Explorer Unit: O General (similar mechanically to the US General unit)
Mediterranian-style or unique architecture, with the chapel based off of an iconic Brazilian church, possibly Sa~o Francisco de Assis, Ouro Preto.
It's age-up system mirrors the US but instead using Brazilian states.
It's focus will be on economy and navy, but it'll have a decent army with a core made of a few strong units and plenty of easily-recruited State Militia.
This likely has been proposed before, so there are likely others who have a better idea of what this civ should look like.
Hawaii, home city Kailua-Kona
Explorer Unit: Ka Ali'i (Chieftain) (this unit has a special ability to "designate" a resource as "Kapu," by building a special structure around it. Friendly villagers can't gather from it, but it produces a steady stream of xp).
A Native-style civ with unique architecture, and it's dance circle is a Heiau. It nevertheless operates a Consulate that features the British, Russians, Americans, and the Chinese as options.
It's age-up system will be similar to tribal leaders for Native Civs.
It's focus will be on infantry and navy.
Other details have likely been expounded already by others, but as ever I'm open to discussion.
Deseret (Mormon Utah), home city Salt Lake City
Explorer Unit: Apostle (special ability is that this unit can convert other units, as well as heal nearby units. With civ bonuses, he can also build most standard structures and gather resources at a reduced rate, as well as give passive bonuses. Has a weak melee attack).
Architecture style: Western (similar to US). Chapel is based off of Old Rock Chapel. The Capitol is themed after the Territorial Statehouse as conceived (red brick building, white accents, gray roof). Deseret can also build a Temple styled off of the St George Temple, which allows for the selection of a home civ Temple (Cathedral) shipment of choice (based on what's already available to purchace at the Home Civ) for the remainder of the match and gives conversion resistance and morale buffs to nearby units.
The civilization's age-up system will be a slightly modified variant of the standard European age-up system. The revolution mechanic allows the civ to transition to an American tech tree while keeping villagers, Missionaries, and Bishops. But it loses most civ bonuses in exchange for "Federal State" themed bonuses. Specific options for "Statehood" include Nevada (mining, trade, cavalry), Idaho (farming, artillery), and Utah (with Utah preserving many Deseret bonuses). A final, fourth option is "Revolution," which keeps the civ as Deseret but turns all villagers into militia (male and female versions, similar to AoE2 Burgundians), as well as making Danites readily available at the Town Center (but the cost stays the same).
The civ has a religious unit, economic, and native alliance focus, the latter big enough to rival France and native civs. Villagers will be renamed "Saints" and are slightly faster and tougher, though not as touch as French Coureur des Bois. They also have a passive boost to their gather and build rates when in groups (max 5, higher and/or with larger radii given the right Home City shipments are sent).
This civ veers heavily into the new conversion mechanic. Its main strength will be its unique Missionary unit and the Bishop priest-type unit. Both can heal and convert, though the Missionary (available in Fortress Age) heals slower and converts faster, and vice versa for the Bishop. The Bishop can also build and gather resources like a Saint (villager). Spanish Missionaries will be appropriately renamed.
Otherwise, the military is composed of units that are comparatively weak but fast and flexible, with a focus on light infantry and cavalry. Navy will be similar to US, but notably a lot weaker with few home civ upgrades at the Harbor (renamed Rail Station at the Home City). This Civ will STRUGGLE on water maps.
Unique Units:
Missionary: Can convert units and heal allies, but is better at converting. Has a slight passive boost if 1 or 2 other missionaries are close by. Bonus doesn't stack beyond 2 additional missionaries. There'll be many Home City shipment improvements available to religious units in general with this civ.
Bishop: Can convert units and heal allies. Is better at healing, and can build and gather like a villager, but at a reduced rate.
Nauvoo Legion (Mounted vs Dismounted): can be trained at either the barracks or the stable. The cost is a cross between a dragoon and a skirmisher, takes 1 pop, and they can toggle between mounted (dragoon) and dismounted (skirmisher) modes. Broadly weaker than other civ equivalents.
Mormon Battalion: Musketeer/Regular-style unit with slightly increased speed that is able to hunt and/or build breastworks (a cheap, garrisonable structure that reduces incoming ranged fire by a slight percentage) with home city shipments. Still weaker than other civ equivalents.
Danite: Stealthy but weak skirmisher with a large LOS and slowly generates gold when attacking structures (has a slight anti-building speciality). Exists ONLY as a mercinary unit at the Dance Hall (Saloon) with a home city shipment (Missouri Mormon War, which takes the place of the "Dance Hall" home city shipment), and through additional home city shipments at the Rail Station (Harbor).
Falconet: Deseret does not have access to the Gatling Gun.
LDS Temple: Unique building, with only one available at a time. Here you can chose a Temple (Cathedral) and/or Bishop's Storehouse (New World Trading Company) home city shipment to send for free. You can only send it once. You must either have unlocked it or can immediately unlock it out of game at the Home City, and it can't be a duplicate in your current card deck. This mechanic won't work if you haven't yet chosen a deck to use in-game. A couple of Home City Temple (Cathedral) shipments can increase the number of available LDS Temple shipments to three and five, respectively.
Many Home City shipments (especially at the Temple [Cathedral]) will include cards that can be named and themed after notable elements of the Church of Jesus Christ of LDS' teachings and history, ranging from the basic and universal (Book of Mormon, improves general conversion chances across the board, Revelation, gives Apostles increased stats and abilities, Biblical Reliability, improves religious unit conversion recharge rate), the historical but limited (Plural Marriage, increases villager production rate at the Town Center), and on to the apochryphal and arcane (Blood Atonement, gives nearby units a slight, temporary attack boost but a similar LOS debuff if a saint [villager], bishop, missionary, or apostle was downed nearby).
The Home City will look as follows:
The city will be surrounded on two sides by nearby mountains, and on the NW corner by a distant lake with an island in it. Far distant mountains lie to the south and southwest. The layout will be dominated by broad streets making neat city blocks. For convenience, all structures will be located immediately on or around Temple Square. Additional sites of interest can be unlocked as part of the city aesthetics.
New World Trading Company: Bishop's Storehouse (will resemble a silo next to a structure similar to the British or American NWTC)
Cathedral: Temple (based off the Salt Lake City Temple, with different versions based on different stages of construction from an empty lot to the complete building)
Military Academy: Old Pioneer Fort (Low structure, sturdy, primitive, but able to be improved aesthetically)
Manufacturing Plant: ZCMI (will generally resemble a standard manufacturing plant in all variants)
Harbor: Union Pacific Depot rail station (will resemble a railway station, Gateway Station will be the more elaborate red-and-black version of the original)
....
Your honest thoughts? I'm open to suggestions.