I always wanted to make a seriously in depth Tactical FPS game. Back when I was heavily into Rainbow Six 3: Raven Shield, I came up with these ideas:

1. Wounding system:
If you are shot in the limbs, then that limb is injured and effects mobility. IE Arms would effect your aiming and ability to hold a gun straight. A leg would force you to limp and/or drag yourself depending on severity of the wound. Also, you would slowly black out due to blood loss, unless a teammate tends to the wound. The blood would drip onto the ground or even smear if you were crawling or dragging yourself along. If noticed by the AI, then they could follow the blood trail to find you.

Teammates would also be able to drag or carry you to safety. If they were dragging you by your shoulders, then you could still fire your sidearm to give them cover. (COD4 kind of did this with the Last Man Standing Perk) Or, you could suicide yourself, by pulling the pin on a grenade and just holding the level closed. So when an enemy found you, you could release it, or drop it when killed.

2. Realistic hit location damage. Like what the Soldier of Fortune games started. By down to the minute detail. Organs, Bones, Tendons, etc.

3. A full breach and rope repelling system. (Which Rainbow Six did with Vegas)

4. Hostages that were intelligent, and could be anywhere from in shock, to fully cooperative, to suffering from Stockholm Syndrome.

5. Being able to subdue and arrest targets, rather than always having to kill them. (Which Swat 4 introduced.)

6. Full pre-mission planning, with not only team members, but also with Helly support, snipers across the street, and even being able to have the city cut power to the building on your command.

Now that I think back on this, I want this game more than ever. I am so sick of these auto-healing, one man army, carrying way too much gear, weapons, and ammo bullshit games we now have. (COD, Halo, MOH, etc.) I want realism, not arcade. I want tactics over action.