I think it'd be really cool for rare structures to spawn in cities like stadiums, landmarks like the eiffel tower, colosseum, or space needle, or themed buildings like hospitals, schools, fire stations, police stations, or restaurants. For stadiums and landmarks maybe one would spawn around once in every 2000m x 2000m area, or only spawn once per world, to avoid seeing multiple of the same landmark, without needing a ton of different landmark structures, whereas themed buildings could be more common.

I would also love to see this and I also think datapack makers should be able to create this very easily already by modifying the citystyle files.

You comments are a little old at this point, but check out the links below -- for standalone (part of 'outside' generation, defined in worldstyles/[style_name].json in scattered.list) structures you can define 3 chunk by 3 chunk structures -- I think you can define specific chunk coordinates for generation, I don't think there's a 'uniqueMinDistance' attribute like there was in ruins, for example.

As part of city generation, if you combined a number of conditions and gave the monuments a low weight, you could probably almost guarantee that players would only see a building once or maybe twice.

docs on the data pack system:
https://www.mcjty.eu/docs/mods/lost-cities/asset-datapack

video tutorial:
https://youtu.be/Ck9_yknPx4c