I'm doing a little research and would appreciate some opinions and feedback.

Let's say you, an experienced Minecraft player, were exploring a server based on a real-world city (that either you knew, or didn't know). You joined, and it contained a copy (a machine interpretation, anyway) of hundreds of square miles of a real city.

If you were able to build in and modify that world as a player, what activities would make it fun for you to spend nontrivial time in that world, and return to it? If you couldn't modify it, but you could explore, is there anything that would make you want to spend time in it?

A flight sim mod could be lots of fun to play with in a huge city map, but I imagine the audience for that is pretty niche.
If I couldn't modify the world, maybe some games that use the environment such as Hide and Seek could be fun with friends.

I think there are a lot of possibilities for both offline and online applications.

As slime mentioned a flight-sim is a good way to appreciate large spans of a city ... just the whole crashing into things may be a little bit of a hurdle to overcome so the right impression is made.

Flying/driving/rail sims would benefit from such large maps in general I think.

Regarding online applications perhaps some form of AR where you can look around while being inside a real city to see what it would look like if it were 'blockified' from your point of view would be neat. Could even be incorporated into a scavenger hunt or digital goods/NFT exploration kind of game too.

If I were able to build in / modify stuff in creative mode, I would probably have a lot of fun blowing stuff up for a bit, so I imagine that if enough people joined, it wouldn't be based on a real-world city for long. Maybe blocks could automatically regenerate after a few hours, which would allow players to destroy whatever they want without impacting the experience for the next person.

If partial modification was possible, where you could build things inside of buildings but not modify the buildings themselves or the outside, there would be a lot of potential for base-based PvP. Maybe various items could spawn at points of interest every once in a while, similarly to Pokéstops in Pokémon GO, giving something for people to fight over.

Even if the world were completely static, it would still be interesting to walk around it or fly through it with elytra. There's probably many minigames that would be fun in a static world, or a world that got reset after every run, but I don't really play those so I can't say much for that.

Sandbox

Definitely use elytra to fly around like I was a superhero. Def place some TNT and blow stuff up too. If it was a private server with friends, we'd get into shenanigans. Maybe a parkour course between the sky scrapers, maybe a tight rope walk, Maybe jumping off one building and crashing "through" a window on another - as best as we're able to in MC - like an action hero would.

Explorer

I'd love to visit historical locations in time and explore. Walk up to places and see a recreation of an event with some dialog or notes explaining what happened here. Sort of like the Library in Minecraft but it'd be hundreds to thousands of historical locations we could visit. I know it's not within the scope of your question, but I just don't see a city with that much current history. There's probably been notable events in most major city over the last 5 years but I don't think it's enough to create an exploratory experience for.

You could toss in historical buildings, facts about other buildings. "This was the tallest building in the city until 1940 when this other building was completed. In 1950 this building had a fire on the 22nd floor which threatened it's structure, but the town rallied and it was restored instead of demolished." I think local history would be cool, just hard to source.

For a world that I could modify, I'd probably have fun trying to transform it into an expansive CTM map. So essentially a scavenger hunt, but having to deal with mob spawners, traps, etc. as is usual for the CTM format.

If I couldn't modify it, I would just enjoy exploring and learning about the city.

The FBI would like to know your location

This is really helpful, thanks, everyone! A few ideas that I had that might spark some further thoughts:

• Claim a building and build a better/more detailed version of that building (including a base inside that building?)
  
• Relatedly, build missing infrastructure like bridges
  
• Create the subway system (that's definitely a Kerm-themed one)
  
• Replace an (unremarkable) real-world building with a much better/cooler one of your own
  
• Scavenger hunt - we're thinking on the same page, JamesV and others

That's really interesting.