I would design a career based modern (or futuristic with antiquated specialization schools and rank systems) shooter. You would begin your career as a Private in the US Army or USNC (something like military in the future) and the tutorial for the game would be a mock of Basic Combat Training (boot camp).
You would pick an MOS or job and run through the same set of missions regardless but with different objectives and different NPC counter parts to fufill the objectives that you can't. Successfully completing missions increases rank and unlocks additional training you can take in the form of side-missions , completing secondary objectives increases it faster. The idea is that with more training and rank, you can control the way each mission goes more and more. If you take Ranger school, you might prefer a light-unit approach by using your Airbourne school training to parachute in a few kilometers out then sprinting in to avoid attention. Eventually you could take HALO (high altitude, low oxygen) for rapid insertion only a few hundred feet away. Sniper school, where you would learn to build guilly suits. Sapper school, where you would learn demolitions, why use doors when you could make them? You would always work with NPC's with the same skills, so as you improve, so does your team. You COULD complete your career (the game) as a plain-jane infantryman if you wanted, but you could experience different parts of the game depending on which career path you take. Sneak in to avoid much fire and complete objective. Shut down the power and destroy alternative entrances to keep out enemy back up. You're being attack from the rear by enemies coming over a bridge, push them back and blow up the bridge! If you just played plain infantry the whole time, it would straight out fire-fight 100% of the game.
Eventually, a specialization school called "selection" would unlock, upon completion, you would become a member of "special forces". You would now be able to use any weapon you want, rather than the usual M4 rifle, M9 sidearm. You could hand pick every piece of your layout. Take any weapon you want. But you would have to consider weight and ammunition. Carry three assault rifles and a M107 .50 cal barrett? Don't expect to move very quickly. You want a sawed shot gun? cool. that one .50 cal full-auto pistol that Steven Segal had? ok. M92 Raffica burst fire? M14 full-auto? M240B? Ak-47? You want it to have a holo-graphic sight? Laser attachment? flash light attachment? Bayonet? Anything. Wear or dont wear however much armor you want.
There would be a life-bar with SLOW regen, but you have a first aid kit to stop bleeding and drugs to ignore injuries, limited supplies however. Because you would bleed or suffer functionality of limbs unless struck directly in your body armor. You would have a visible "stamina" bar which is drained whenver you use a melee attack, scale a wall, jump, sprint, receive damange, or use large caliber weapons. You would have invisible meters for all of your performance throughout the game increasing your capabilities. The more you sprint, the longer and faster you'll be able to sprint and the more you'll ignore the encumberance from your equipment. The more times you come close to death (screen fully red) yet survive, the more damage you'll be able to take in the future. Increased accuracy on your behalf is slowly rewarded all your guns being just a little more accurate or increased speed looking down the sights. Use of weapons with lots of kick will eventually reduce the amount of kick you experience from all your weapons and increased speed with maneuvering all your weapons.
The actual story line would be invasion of another country. Taking out cities. Assassinating high profile targets. Sabotaging enemy convoys. Blowing up bridges. Team members can die (permanently). The war, that lasts 10 years and spans over several countries, would be the game. And it all ends with a huge invasion of the enemy palace where you chase down and attack the head honcho who NARROWLY escapes.
upon completion of the war, you retire. however, IF you unlocked and completed "selection" and make O-7, Colonel, there is a bonus mission, where you quietly insert and assault his land stand (an island, a sky scraper, a huge oil refinery) and you have a time limit to clear the enemies and take him down or he'll narrowly escape a final time. If you win the mission, he'll be lying on the ground bleeding to death jabbering away in his own language and you get a cut scene where your character takes your weapon of choice (whatever weapon you used the most in the game) and pumps three rounds down his throat.
