There is a class of video game I really like. It's the games where you explore and interact with a city.
For example, the Saints Row games and Sleeping Dogs are about interacting with the city. You can call them "GTA clones", and I guess you can even play them like that, but I don't. I love wandering the city on foot. If you've never been sucked into that kind of play, this essay may be uninteresting.
It's not just GTA clones. Crackdown and one of the Naruto games both featured city exploration/interaction beautifully. On the other hand, many GTA clones (and the GTA series itself) are not really about city exploration in this manner.
Done right, the gameplay I'm talking about feels like a combination of mountain climbing, treasure hunting, and immersion. Cities are amazingly interesting places full of interesting terrain, nooks, and lived-in areas. I've played a huge number of Saints Row hours, but I can't get into GTA at all, because GTA doesn't provide that kind of play very well and Saints Row excels at it.
The big problem is that getting on a vehicle basically ruins that kind of play. You can't climb mountains, treasure hunt, or see the details of the city very well from a vehicle. But the GTA "genre" is theoretically defined by your ability to steal vehicles.
You can steal vehicles in Saints Row and Sleeping Dogs and Crackdown, but it's really not an interesting part of the game. The weakness of a vehicle game is that it has to spiral. You get a crappy car, then a better car, then a helicopter, then a jet, then a VTOL, then a superbike that can fly... mobility gameplay is one that spirals - you get more and more mobile.
On the other hand, city exploration and interaction doesn't. You can explore new areas of the city using the same level of mobility you start with because the point isn't your mobility - the point is the city you're exploring. By simply moving to a new part of the city, you uncover new architecture, new people, new shops, new hidden treasures, new sidequests, maybe even new enemies and NPCs.
This is why I played so much of these kinds of games. I literally spend hours just wandering the city.
So what I'd really like is a game where this is taken to a new level. Let me have fun designing a game in broad strokes.
This is a roguelike where the city unfolds as you explore it. Since there is no natural cap on exploration, there's no reason to put a cap on it by having a limited city. In addition, older regions can go "under construction" and go into a construction-zone style, then re-emerge as a new zone.
The key is that the city zones have to be designed to be interesting to foot exploration. This means variable height, somewhat-climbable buildings, collectables and unique shops.
Unique shops are really the major factor here. The replayability of games like Saints Row and Sleeping Dogs is limited by the amount of content you can discover. There are only so many costumes you can wear, only so many weapons you can try out.
But unique content is not impossible by any stretch. Generative content can create an infinite variety of clothes, guns, and vehicles. Different structurally as well as just in terms of color and texture. Eventually you'll encounter the full variety and effectively "get used to" the content, but it'll be an order of magnitude more bang for the buck. Wear mechanics that steadily destroy the unique content as you use it are also useful.
The other major factor that needs to be brought into play is NPC reactions. In most games, content is pretty much used by you to your own preference. However, there are limits to that - your taste is specific and that writes off the vast majority of the content. Therefore, the content needs to have a very strong game effect. Normally you could make it combat-related, but in this case the exploration/interaction mechanic is more important. Therefore, your gear should strongly affect your ability to explore. In this case, I would make it so that certain types of people will help you/let you into their buildings if you're dressed in their preferred manner.
Actually, sharing with NPCs is important in general. You can only ever be one person wearing one thing, driving one car, shooting one gun, using one dance choreography, eating one pie, living in one hourse, whatever. By introducing NPCs that hang around, you allow the player to equip them or influence their choice of equipment.
You have to be careful not to make them burdens, but in this kind of gameplay that's actually pretty easy. They can hang out, follow you around, show up randomly, whatever - as long as they don't actively get in your way when you're trying to jump around, it's all good. You can even use them to help you explore, giving you a leg up or distracting a nosy cop or something.
I'd play that game... basically forever.