Call of Duty 4: Modern Warfare (hereafter COD4: MW).
If COD4: MW is highly moddable then the skilled people that did big-mods for STALKER need to grab it and make a total conversion to a sandbox game. My time and inclination for doing such things has passed so I live in hope of someone else providing (sad as it is). Unfortunately I’ve read that the COD4: MW engine uses spawning to create enemies. I’m sure that can be rectified. Spawning is the elephant in the loungeroom of FPS games. The whole concept is flawed. Everything that was not already in the environment has to come from outside the environment. It doesn’t have to ‘fully’ exist in the sense that it is rendered but it should be reacting to the environment with AI. The environment should also be reacting to it! (see Sandbox Shooter: Emergent Genre) This can be done algorithmically or simulated: doesn’t matter as long as the new enemies/allies/critters come from outside the environment and then work their way in. It may involve them being spawned at certain points but with thought it can be done with internal consistency and not look like the kind of cheap-magic-trick that it is in most games.
Consider terrain as borders. A tunnel is a great example. Prior to more ‘bandits’ being spawned an animation of torches coming down a tunnel can be a logically consistent warning that ‘bandits’ are about to spawn at the mouth of the tunnel. I digress into sandbox design, again.
COD4: MW has all the game engine elements that would support the ultimate Sandbox Shooter with today’s technology. Playable wounding (if you don’t fall over, then you’ll survive if you’re not wounded again for a number of seconds), fantastic ballistics (allowing easy penetration of cinder-blocks with 7.62mm weapons for example, 5.56mm has a harder time and pistol rounds don’t), great environments, and good sound-engine.
If I was to pick any engine that existed it would be this one for a sandbox shooter. Rendering is excellent and the physics engine is fantastic. There’s rumours of a sequel in October this year that will add RPG elements. The unfortunate thing is that the environment is not dynamically destructible. I’ve seen shooting of a single cinder-block with over 60 rounds of 7.62mm and the cinder-block is still there: it should be gone. If the engine could cater for destructible environment it would be the perfect base for the Sandbox Shooter.