they should really explore fusion districts that replace multiple districts or combine them. Arabia’s Madrasa, Korea’s Seowon, Maya’s Observatory, Germany’s Hansa, Greece’s Acropolis would definitely fit in this category
Kinda expanding upon this: what I would personally do is
Turn the Madrasa into a district replacing the Campus and Holy Site. The Campus is the ‘primary’ district, so it inherits the campus’s buildings as well as the temple, but it also generates faith and great prophet points, and is all you need for missionaries and apostles after getting said temple
The Seowon is a school of legal studies irl, so I would make it a campus replacement with the ability granting you a policy card the first time one is built + another for your first university. All campus buildings also grant additional culture.
The Observatory kinda falls in the same line as the Madrasa, but doesn’t replace the holy site. It grants faith once a library is built, and an additional temple can be built within the observatory.
The Hansa fully replaces both the Commercial Hub and Industrial Zone. You get 1 trade route and +4 gold for each building within the hansa. +2 adjacency on both production and gold if it’s built adjacent to coast.
The Acropolis would be a full replacement of the holy site and theatre square and fully replaces both. +2 adjacency on hills, and you get access to both sets of buildings
Other potential fusion districts that come to mind, as well as the civs that could use them would be the Jantar Mantars, if they ever wanted to do a Rajput civ, as a campus/holy site replacement.
The Kovil could be a cool Holy Site/Neighborhood fusion for the Chola
Barays could be a Aquaduct/Dam combo for the Khmer
A Mughal Fort could serve as a fun Encampment/Theatre Square combo
Hittite Forge Complexes could be a cool Industrial Zone/Campus combo
As for now we can say that both Harbor and Encampment are multi-use, in order to offset the lack of production in coastal cities/the lack of city infrastructure when you go to war early. Recently, SS buildings such as Gilded Vault and Vampire Castles are also lean to a similar role (compensate for lacking of culture or production).
Harbors and encampments aren’t what I meant by fusion districts just bcs rather than being the combo of two districts, they just are multipurpose by nature.