Thought I'd see if I could jot down a couple more ideas + a bit of justification for each. This isn't necessarily a list of Civilizations I'm desperate to see, of course, just hypotheticals.
Exploration:
- Umayyad: Khirbat al-Mafjar
The Dome of the Rock is definitely more well-known, but with the simplicity of Civ VII's religion system, I thought that something else would likely make for more interesting gameplay. Khirbat al-Mafjar is one of the iconic Umayyad "desert castles," and even centuries after it has fallen out of use, it demonstrates many components of Umayyad art, architecture, and technology, with its many surviving frescos and evidence of the hydrological systems that fed its baths and farms. My first thought is a sort of Exploration Age Petra equivalent, but any food, culture, or happiness-related bonuses would be appropriate.
- Hanseatic League: Marienkirche
The St. Mary's Church in Lübeck served as an inspiration for the spread of the iconic "Brick Gothic" style around the Baltic, and is thus representative of the city's wide influence as head of the Hanseatic League. In addition, with the tallest brick vault in the world, it stands out as a representative for the style even disregarding its age and historical context. It was also built directly adjacent to the city's merchants' quarter, so all in all it could reasonably be given any number of abilities related to trade, diplomacy, culture, or production.
- Khwarazm: Najm ad-Din al-Kubra Mausoleum
One of many mausoleums standing in the ruins of the empire's old capital, Najm ad-Din al-Kubra Mausoleum is named for an influential Islamic scholar from the empire, and could thus be given religious, cultural, or scientific bonuses. However, its location as a whole is particularly interesting because it was a center for the area's signature architecture for a very long time, a home to many beautiful buildings and a place from which prolific architects were often recruited (for instance, several buildings in Samarkand were created by builders and architects from here), with the construction of monuments continuing even under its Mongolian conquerors. Those two ideas, Islamic academia and inspiration for further wonder activity, make me think of an ability similar to Aksum's Great Stele, giving you culture or science for building wonders (both helping you with general advancement and with unlocking more wonders to build.) Frankly, Khwarazm is a civ that's not too high on the list in what is a very competitive region, but even absent them I'd really love to see the old city of Khwarazm itself as a Wonder with several of the notable buildings (including this one) visible in the model (the rationale for the ability would be the same either way).
Modern:
- Peru: Museo Larco
There are many stunning colonial cathedrals in Peru (if I were to pick one for the wonder, it would likely be the Basilica and Convent of San Francisco), but it seems that the go-to wonder picks for all the other Latin American countries also lean heavily towards that style (even in the base game, Mexico has the "Catedral" as a UB), and with the idea of Peru as a civilization focused on archeology and natural wonders stuck firmly in my head, I wanted to go for something that very explicitly tied into that instead. Hosted out of a former government building from the 1700s, Museo Larco has an extensive collection of artifacts from pre-Colombian Andean cultures, which obviously lends itself to abilities related to the cultural victory path, but I could also see it granting bonuses to unique tile improvements or buildings from previous Ages, perhaps allowing you to excavate Artifacts from them? Or perhaps that would work better as a civ ability... either way, I think it does what it needs to as an associated wonder. If none of that sounds good, I'd say that the Monastery of Santa Catalina de Siena is quite beautiful and would be my second choice.
- Finland: Helsingin Yliopisto
The University of Helsinki has a storied history and was greatly influential in the modern history of Finland. I could see it getting any manner of bonuses focused on culture, traditions, science, and the like.
- Imerina/Madagascar: Ambohimanga
A traditional Rova (fortified royal settlement,) Ambohimanga played a major role in the Kingdom of Imerina as the seat of several important monarchs (as well as holding many of their tombs) and remains a significant symbol of Madagascar's culture and history today. It could have bonuses focused on culture/traditions/happiness as a modern symbol and as the former official "spiritual capital" of the nation, or could be taken in a different direction entirely and have militaristic bonuses based on the campaigns that were led from it.
