As a game environment, I'd just use an area of the giant city as a sort of a hub or a "break area" between levels, that the player can come back to explore and get upgrades and new missions. Kind of like the village in Zelda: Links Awakening.
I personally wouldn't have the main levels set in a city (unless its a destroyed post-apocalyptic city) because IMO realistic city levels are difficult to create. In a real city, each building, store or house looks different and it would be too much work for one person to create hundreds of different buildings. And it might just end up feeling repetitive.
My idea to make buildings feel unique is to have them be randomly generated from parts that can be combined in multiple ways, so even if two buildings has exactly the same sprite set, they could look different if they have different shape (and even with the same shape, they might be different heights). I've done a bunch of research on this, like studying how EDF 4's city was generated (one of the fun tricks they used was making buildings whose short and long sides look different, and then turn some of them 90 degrees) and reading through Shamus Young's blog on the procedurally generated city screensaver.
It's all about movement.
GTA cities don't work because there's things hidden on it, that's a bonus. They work because you're driving everywhere. Drivin is fun, and you need big places for driving to be worthwhile. The place is huge, but you can go from one end to the other in a pretty damn short amount of time. This is vital for a huge city to work. You could find fun in GTA even if there was literally nothing to do in the map, even if it'd be pretty short-lived.
Another good example. The newest Spider-Man game has a massive city with very few points of interest relative to its size. The whole thing is seriously near empty, and outside the story missions and the randomized crime, there's objectively speaking just not a lot to do. It's also one of the most fun you can have in a gaming city because navigating it feels Amazingā¢. Here, you could also have fun even if there was literally nothing to do in the map. Perhaps even more so, as you quite literally need to interact with the environment itself in order to move, and the layout has a direct impact on your options.
A bit of a counter-example, Super Mario Odyssey has a complex movement system and several different moves you can chain together, but the system is not really tied to speed, but to making sure you get exactly where you want. Since you're not that fast, the maps are a heck of a lot smaller, but they still managed to add a level that feels like a large city, and they had a fun way to navigate it.
Don't worry about content. Worry about making the journey from point A to point B as fun as possible. Make the movement system compelling, give it a high skill ceiling. Reward good players with more speed. If you do things right, good players get the satisfaction of getting to their objective quickly, while bad players get to have fun learning the system and getting better at moving.
Now you just have to worry about using a system that makes sense for your game, but that's really the most important thing. A fun movement system with a high skill ceiling can make any city fun. If navigation is not both interesting and effective, there's really no way to justify a large city. If traveling takes long, it feels tedious. If traveling is effortless and trivial, it feels boring. If traveling encourages you to get better at the mechanics and doesn't waste your time, you're gonna get hooked.
Some really good points here! The game would be a turn-based strategy game, so I've had overworld movement as an afterthought... but I guess I need to think more about how I can make it fun, not just the boring filler between the fun parts.
Off the top of my head, I'm thinking being timed to get to places and get rewards for getting there early would make sense a lot within the game's theming - you know, when someone is being robbed or shot and you need to get to the crime scene while the trail is still hot. You get approximate coordinates and need to get there as quickly as possible, you get to decide whether you wanna parkour or get to a vehicle. Getting there quickly gets you some sort of bonus... extra money for a job well done, bonus EXP, a free first-strike turn because the enemies were unprepared, something like that. The game is set in the same universe as my big platforming game series, so I guess you could do a bunch of random parkour moves (wall jumping, jumping several times your own height, dashing) and I probably could work in one of those hang-gliders like in Breath of The Wild without too much handwaving. Maybe grappling hooks as well, I guess.
Maybe I should forego the traditional menu-based movement altogether even in battle, and have you move around using the normal overworld turn mechanics (within a cylinder that represents your movement range or something) so the fun isn't taken away from you once you get in a battle? Not sure how much it would add other than a more tactile feel, but i guess it could open up new strategical situations like being able to get to places outside your normal movement range (e.g. high ledges) if you're good at all the different kinds of jumps?
Some thoughts i have about potential "zones":
o Central Park type area- a given to have some wilderness in the game, though you could of course do smaller ones if that's too big.
o China Town, Little Italy, etc - easy way to add some distinction and flavor, also you can have different mafias controlling each area. If your game isn't as serious, you could make something silly like "kiwi town" run by the new Zealand Mafia.
o Underground transport/sewers - lots of interesting things going on there
Don't worry, I have this covered! The city would have 5 major districts (old town, financial district, old slums that are getting posh now but have a nightmarish street layout so you get lost easily, modern villa area, modern slums that are still under construction), several major unique landmarks, enclaves that are "mini-districts" (little japan, little india and little USA being 3 of them), and there's gonna be a lot of subway stations... perfect way to get fast-travel points and stores to restock on stuff to be conveniently close everywhere.
I feel like there is 2 different questions being asked here. "What is the advantage of a giant map?" And "What is the advantage of having every building have an interior?"
I'll try to give ideas for both.
Advantages with a giant map:
1: Like others have said, space for satisfying travel mechanics.
2: Like you mentioned, Destruction of buildings.
3: Random repeatable quests. Having a giant map gives you more spots for randomly generated quests to take place.
4: Random Events. Similar to the above but for random events. Things like crimes occurring in the world.
5: Territory. You could have gangs or factions have their own territory. Or you could have player owned territory.
6: Sense of freedom.
Advantages with having every building have an interior:
1: Like you mentioned, rare loot or collectibles.
2: Things to steal. similar to the first idea but this is specifically for bad guy characters giving them opportunities to steal things.
3: Random repeatable quests.
4: Random Events.
5: Simulation of individual people. This is most likely too time consuming or too demanding on performance, but you could have NPCs have their own life. Their own jobs they go to
and places they shop and houses they own. Again this is a unrealistic idea to implement.
6: Sense of freedom. Even more so with having every building having an interior.
7: More hiding spots. If for example the player is running from the cops they will have the option of breaking into a house and hiding there.
8: Better escape. Similar to the above. The player can break into several houses weaving in and out of them to more easily escape the cops.
9: Cover.
Not sure if any of these ideas will fit your game but you be the judge.
Good call on the split! I had some problems narrowing down my question, but I couldn't tell WHAT was the problem... turns out it was two questions all along. Thanks for the enlightenment! ^__^
Giant Map 1-4 has been covered already, and arguably 6 as well (being my core idea). I like the idea of territory, though... I've played around with simulating gangs (and other entities, like all major story players like companies and bosses you'll meet later) so you could get info about current events, and going to a place one of them takes place
while it takes place would have you witness it (e.g. a gang war) but I didn't think about gang TERRITORY yet... but that makes so much more sense than simulating multiple interconnected schedules! Or at least, is a lot easier to represent on a map...
Inside a gang's territory, you have a higher chance of random encounters with members of that gang (combat or peaceful - NPCs that spawn just have a higher chance to be gang members), if two adjacent regions belong to different gangs there's a higher rate of random gang fights,
killing defeating enough gang members will weaken their grip on the territory and lower crime rate, gangs will take over neighboring territory if left unchecked for long enough (crime rates go up EVERYWHERE over time because you're the only non-incompetent cops in the entire city)
Have missions they are not just (go to these coordinates) but more like "go to this bar" or "find such and such person"
And the then you can ask people on the street answer the bar is and they give you directions, or all ship owners of they have seen the person.
This can make for interesting gameplay is you add rules on top ofthat. Like is you are looking for an alcoholic you would go into bars to ask if people saw them.
Good point on the "go do this, not gonna tell you WHERE" thing - I remember The Sinking City did that a lot and it made you really FEEL like a detecive, so I'll definitely steal that idea! The city would be on a Manhattan-style grid (to make coding how it works easier, mostly) so it would be possible to just give people simple coordinates (3rd avenue 25th street) as an easier challenge... or be more mean and tell them to go to "Robin Hood Mall" or "the wooden house on Long Island" and force them to do a lot more research. Not sure if I wanna go so in-depth that I simulate what different people know about different things, it's starting to feel like feature creep / simulation addiction, but it probably wouldn't be too much work to do that for scripted encounters... having designated people that deliver clues randomly spawn until you find them, and have list of WHERE they can spawn... specific places or types of areas.