I have quite a few ideas.
* Very realistic Old West FPS. Your typical "one lucky shot to kill" style of realism, but with the kind of graphics and physics engine we expect in a modern game instead of looking like an ARMA reskin (and much better controls that allow for fast snap shots, rather than the old ARMA and Operation: Flashpoint style with the floaty controls). Plus a good, realistic damage model and well-modeled ballistics for the bullets. Basically, look up any of the classic Old West gunfights and think about trying to recreate that style in a video game.
* World War I tank simulator. Especially a multiplayer one where you can have real humans in all of the crew positions.
* Something similar to Battle Royale or The Hunger Games: dozens of contestants dropped in an isolated area with randomly distributed weapons, devices, and supplies, all of whom need to kill each other until one is left standing. I actually created a very long treatment for this concept that's currently on my old computer, including a gigantomungus weapon list. It also included the aforementioned realistic ballistics and wounding system (including using physics calculations for the damage instead of just punching numbers; think an object damage system for the human body instead of a wall), a first aid system based on using real life knowledge and techniques for different injuries (with weapons and hazards designed to inflict various injuries, such as vials of acid or poisoned blades), a destructible environment that even extends to chopping down trees and shattering windows to damage with the shards, and a single player mode with randomly generated "contestant" personalities so they can negotiate, work together, and deceive.
