Could be anything really. It's already been said on the PS Blog that Ratchet & Clank will have it's first gameplay shown this Fall, so that will likely be shown at the December 2014 event Sony are hosting. I doubt that's what this rumor is referring to.
I wouldn't be shocked if GT7 finally gets gameplay shown at that event too, since it could be a 2015 title and new stuff from Uncharted 4 will no doubt be shown as well.
Reveal could mean gameplay being shown for the 1st time of a game already announced, but if we're talking about a new game unveiling then there's plenty Sony could have, like anything from a 1st party studio that has yet to show any games for PS4.
I hope it's not something that was multiplat, but is going PS4 exclusive, even though I am a PS4 owner it just causes too many ructions and hate within the gaming community.
Now some new IP from a big 3rd party studio is fine, even a spiritual successor to something to an existing franchise (as with Bloodborne) is cool by me.
The reveal of Agent for PS4 would be awesome.
Personally I'm really intrigued to see what Sony's 1st party studios are working on, maybe Santa Monica's new IP wasn't really canceled and they're going to reveal some huge, futuristic Sci-Fi game, with an epic God of War style, set in an open world, with fully destructible scenery.
Like a Gods of the Future or something Marvel/Thor inspired and that's where all the Norse/GoW rumors have come from.
Maybe Horizon will finally be unveiled and it really lives up to the concept art that leaked.
