For sure it will be something using VR, >>CopyPasted>> Half-life revolutionised first person shooters and storytelling in games by never dropping you out of the first person view and allowing you to interact with dozens of things during play. Instead of all npcs being enemies you had friendly npcs like the doctors, you could sprint, jump, climb ladders, crouch, you had to solve puzzles, take leaps of faiths across chasms, crawl through air ducts and hundreds of other things that modern games now take for granted. Whilst it wasn't the first real 3D fps (Quake being the first), it was the most ambitious, and it's constant on-going story-telling that was only seperated by loading screens with no 'levels' involved. It didn't feature any of the usual 'fight your way through x amount of enemies to end of level then face boss' mechanics that games like wolfenstein, quake and doom were known for, it was a seamless trek through an enormous facility littered with all manner of deadly traps and enemies.Many things you see now in first person shooters as a standard thing was started by half-life, the same as many of the standard things we see in first person shooters today were also started by half-life 2, when it introduced the idea of real-time physics including bouyancy, gravity and ragdoll models.And it's these reasons that I believe Half-life 3 is still being kept in the dark. Valve revolutionised games with HL1, then they did it again with HL2, it's a benchmark to which all other games have to compare themselves, so until they have enough new and unique features to put into HL3 to do it again, Valve won't let anything slip through the net.
Also I wish RTS games make a comeback and get games executed similar in fashion to red alert 2 and aoe 2