So it has to be a game that is:
-The part of a series
-Not from Sony
-It has already been rumored to be coming or at least was wished by the PS community but never got released
-It's not a port
-It's a pretty hig frnchise, or at least a very well known one
The Last Guardian. I know that most people throw that out like FF7 remake or something, but hear me out. I haven't read the entire thread, so I don't know if anyone's made a real case for it. But to me its the perfect fit.
Let's take your requirements, just cause its the latest argument.
-part of a series - Many people see Ico and Shadow of the Colossus as a series, even though neither game is directly related to each other. There's a reason why the game was dubbed Trico...its the 3rd in the series.
- rumored / wished by the community - we all know the story...it was shown, and it went on nearly a decade hiatus. Everybody wants it, but its been so long folks think it doesn't exist...even when Sony clearly says it exists. How many times has the community made an outcry for it...how many times do they have to field those questions.
- its not a port - it qualifies cause technically it never released on the PS3 platform. We all suspect the project has migrated to the PS4. No one at Sony denies it, and its been alluded to in interviews. No one's going to put a port on their cover.
- its a big franchise or well known - speaks for itself. its infamous because of the teams' credibility and its absence.
- not from Sony - not really sure why this has to be...but while the team is part of the Sony, Ueda is not...he's contracting.
Now look at the hints from OPM.
"Why is this a down?" - The Last Guardian trailer began with a feather softly floating to the ground. Feather = down. Who uses 'down' like that in a sentence anyway.
Another thing to think about is that while TLG was announced and shown...no one really knows what the gameplay is like. As far as I know, it never really graced any magazine covers. Its the perfect time to reintroduce the game. You don't really need to unveil it at E3, you've done that already. You feature it in a mag article...let people ogle over the pictures and preview accounts...then show the game in action at E3.
They're only going to show the game when its a guarantee of its release and that release is only a few months away. If I'm right and its shown...it'll be this year. Course Sony's always good for a March surprise lol. There's no series that released in the PS3 generation that would garner lines like the 'most exciting in OPM history.' There's only a handful of teams that rile up the media like that...