First or third person shooter like Hitman or Assassin's Creed, but set in the 80s or 90s. No electronics to keep it simple, I never liked electronic gadgets in games. Two or more secret factions fight it out on crowded streets and buildings. The agents are undercover and the goal is to eliminate the other faction's agents while causing as little disruption to the public as possible. So stealth is important and identifying the enemy agents would be a big part of the game.
During the preparation for the match, you'd select your gear like clothes, weapons and tools. You can hide bigger weapons under more clothes, but the same rules apply to the opposing team, so baggy clothes and trench coats would raise more suspicion than lighter, tight-fitting clothes. Especially if out of place, like on a hot map full of tourists.
The first phase of a match would be spent walking around and trying to identify your targets. There could be some additional objectives to act as points of interest. You can do various things to blend in with the crowd like in Assassin's Creed. No two characters in the game would be the same, so you'd have to keep track of which characters you keep seeing, which ones are acting inconsistent, or keep appearing near the points of interest. Paranoid schizos would enjoy this part of the game. The environment would be interactive and dynamic like in Hitman, so you could cause some events to divert attention, or gain access to some places, or even cause an accident to happen to the enemy. There would be some unattended parts of the map like the sewers, rooftops, maintenance access, where you could use melee or suppressed weapons without alerting the public. You can stash guns or other objects for later use, if you expect to be searched or something. Once a few minutes some new intel would become available, but you'd have to use a public phone, a cellphone, or an earpiece to receive it.
The second phase would be optional, and only after an open gunfight breaks out. The crowds disperse and the police arrive. There are massive penalties for killing civilians and cops, maybe even to the point where it would be better for a player to get arrested than to fight back. Let's say the secret factions are powerful and can pull their strings to get you out, like the Illuminati or whatever. So during the second phase you'd have to retreat to the unattended parts of the map, and do combat there. Gun fights would be realistic and slow pace
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