Something I've been thinking up is not really the game itself, But the way your control it/get immersed.
So here is my idea.
SAO is Sword art online (Since i say it a few times in the description)
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A VERY basic grass area (Looking all pretty and realistic) Just flat, No hills (maybe the sky box can have some) And some walls that you can see when you get close to them (To stop you from bumping into the real ones.)
Now. How the game play will work... First, You would need a warehouse (The same size as your level (Of course this would be no problem since you can just change the flat levels design))
Then you would need a decent scan of yourself. (+Every movement you could ever make) So a 3D model.
Something like the Oculus Rift. For visuals.
sensors for your hands fingers toes (Basically a modeling suit) So something can track your movements exactly.
Maybe some sort of wind device (To simulate wind)
A fake weapon (In the real world, So it feels like you are holding something "I don't know how to simulate feeling, This isn't SAO >.>")
And (Optional) Someone to save your ass just in case you are about to walk into a real wall. ((((This is what the In-Game walls should do anyway))))
Okay, Now onto GAME PLAY
Walking would be as realistic as possible, (Making the In-Game model copy you precisely) Same with running, crawling, rolling, farting and sneezing etc etc.
Since we are making you carry a fake sword/gun/weapon<<< We could render a beastly awesome one in game to match maybe with fire coming out of it. (If its a sword that is, "Maybe i am thinking of SAO Q.Q")
We could Indefinitely have other players (They would have to wear the same silly getup as you though)
The OVERALL idea would be. While INSIDE the game you can get a menu (That opens the same way as SAO farrrk) Or through some other command like a button on the Occlus Rift, Then you can build a fantasy world inside the flat area, (Now we are going garrys mod....). Of course, Since you are walking around with a real body, Going up and down wouldn't work so well But building a huge castle with one floor would still be awesome. (Or something like that)
The best part about it would be, It doesn't have to be (Flat land on the earth!!!!) It could be a huge floating island with waterfalls and stuff too. The overall idea for the level would be yours. Same with how you can interact with it, If we could implement a suit with force feedback, we could even have it feeling like you can punch through walls
(Eg if you punch a OBJECT the force feedback will stop your arm on the object) Causing the object to crack, or shatter or turn to dust >:, Of course you don't want to be punching the ground, cause that would ACTUALLY hurt your hand xD
But you could also build a level that just looks pretty (I know this sounds VERY unrealistic) But you could use your house as a reference model and wear the suit around your house. then you could basically have augmented reality for you house "On a more realistic note this WOULD be doable but would probably take you ages to get the house right.)
This can be expanded in so many different directions, And i don't want to make a TL:DR post. So I'm going to stop now. But you guys catch my drift right?