My game interests is basically open world and racing games. Realism is crucial and it'll be mostly PC games.
When playing a gta game and you're riding in a car, for instance. You see another car out your left side. Why is it that the car magically disappears when you look away for a second?
Imagine a 7km * 7km open world game where oceans, lakes, forests, little deserts, a city and a total npc population of 1,000,000+ is present. Isn't it possible to have a system that saves what happens because in all gta games i've played, driving through an area isn't that fun because let's say you could hit down a tree, if you came back after 30seconds - 3minutes later, you won't see a trace of that fallen tree or cars that got hit by the tree or the bodies of the people that died and that isn't realistic.
Think about that for a minute.
About 50 -- The number of characters on screen at any time for most games.
About 5,000 -- the maximum number of characters allowed on a single server in many MMO games (which are optimized for that sort of thing, and usually involve multiple computer boxes as part of a server cluster).
About 1,000,000 -- The number of characters you are asking to simulate on the player's PC.
Being "realistic" is an interesting goal. The more data you keep around the more processing you need to do.
Consider that fairly complex games like The Sims actually do simulate every person who is active in the world, although they do their best to kick people out of the world when possible. In The Sims 3 you can get around 150 people to arrive in your lot using cheats, and there are also game mods that remove the limits. Getting a few hundred sims active in the same area is enough to bring most machines to their knees.
If you are spending all your time simulating what a million other city residents are doing, and simulating millions of televisions being on or off, cell phones being used, broken and unbroken windows, millions of vehicles with potentially broken windows flat tires and gunshot damage and more ... well, I'm not sure the common home computer has that kind of processing power for an interactive simulation.