I've always wanted an open world game with the scale and attention to detail of a GTA where you play as a deep undercover cop or FBI/CIA agent. The type that go extremely long stretches before a check in.
Ideally, peaking an enforcement agency you work with would be like picking an origin or something like that, and would determine who you're trying to infiltrate.
Ideally, there wouldn't be a morality system, or anything like that, or writing that funnels the main character back towards always deciding that their dedication to the law is most important. You could decide to eventually work up to getting your target taken down through the investigation, flip entirely and just become a part of the organization for real, go rogue to get shyt handled yourself, or just focus on getting yourself out, regardless of what happens.
Depending on what you choose, you'd have to work on balancing whatever real life your character has (you'd be able to customize their background, to an extent), along with keeping your target and employer happy.
Missions wouldn't progress linearly, but would instead be available based on your relationships with mission giving NPCs. So, for example, you could skip an entire minor mission chain that helps establish your cover and prove yourself to the org your infiltrating if you take actions elsewhere that deem that unnecessary. But at the same time, establishing that trust increases the demands of your handler, since they know you've got leeway.
Character builds/progression would help determine what you're good at, and what service you provide to the org is. For example, if you're great at boostong cars, or a great getaway driver, you're not going to be asked to go clap someone in their sleep, since that's not what you're good at.
Game would have failstates built in where your character can get killed or arrested for blowing their cover, or getting burned by your handler after fukking them over. Would also have mechanics where if you're balancing both roles and get arrested, you have to find a way to earn back trust after walking on light or no charges so people don't think you're a snitch.