http://www.psxextreme.com/ps3-news/4796.html

Taking a look at the original source for the rumor the following things have been stated about the game:

1. It's a game that people have been waiting for the announcement of.
2. It's a game the author of the article considers a system seller.
3. It's an unannounced game.
4. It's not Team ICO's game.
5. It's not a remake of Final Fantasy X-2
6. It was mentioned in the first 73 or so comments on the original article.

It's worth giving insight to what the article's author considers a "system seller". In the article he clearly considers the following games system sellers.

Uncharted: Drake's Fortune
Metal Gear Solid 4: Guns of the Patriots
LittleBigPlanet
Killzone 2
Uncharted 2: Among Thieves
God of War III
Gran Turismo 5

Later in the comments section he is asked why he didn't include Resistance: Fall of Man, Resistance 2, White Knight Chronicles or Final Fantasy Versus XIII in the article and he responds that he doesn't consider those four games system sellers.

The 5th statement about the game narrows the possibilities down to 30 games:

1. Twisted Metal
2. Syphon Filter
3. Ratchet & Clank
4. Jak & Daxter
5. Sly Cooper
6. Killzone 3
7. Resistance 3
8. Colony Wars
9. SOCOM
10. The Getaway
11. Heavenly Sword 2
12. Warhawk 2
13. Final Fantasy VII
14. Kingdom Hearts III
15. Bushido Blade 3
16. Onimusha
17. Mega Man
18. Breath of Fire
19. Darkstalkers
20. Metal Gear Solid 5
21. Dance Dance Revolution
22. Suikoden VI
23. Grand Theft Auto V
24. Untitled Rockstar game
25. Earthworm Jim
26. Battletoads
27. Ridge Racer
28. Hitman
29. Gears of War 3
30. Halo 4

Of those games, some of them can clearly be disregarded. Halo 4 is an impossibility. Mega Man, Earthworm Jim, Battletoads, Breath of Fire, Hitman, Darkstalkers, and Colony Wars (All mentioned only once in the same comment) are all extremely unlikely to fulfill either the first of second statement about the game. There is absolutely no way Killzone 3  could be announced or in development so early. Resistance 3, Heavenly Sword 2, Warhawk 2, Ratchet & Clank, and SOCOM are all unlikley due to the fact that the author doesn't consider any of the games' predecessors system sellers. I think we can all agree on the unlikelyhood of Gears of War 3. The Getaway has already been announced, even if it was eventually cancelled. Finally, Suikoden VI, Dance Dance Revolution, Bushido Blade and Ridge Racer all are very unlikely to be considered system sellers by the author's opinion.

Narrowing all those choiced down, we come to 10 likely possiblities.

1. Twisted Metal
2. Syphon Filter
3. Jak & Daxter
4. Sly Cooper
5. Final Fantasy VII
6. Kingdom Hearts III
7. Onimusha
8. Metal Gear Solid 5
9. Grand Theft Auto V
10. Untitled Rockstar Game

Of the narrowed down games, we can nitpick. Jak & Daxter, Sly Cooper, and Kingdom Hearts III are all pretty unlikely due to the fact that each of their respective developers are fully preoccupied with other projects. Onimusha is relatively small time in the big picture plus Capcom is extremely multiplatform oriented. Finally, I just can't see how Take-Two execs could get away with making GTA5 exclusive, because the company shareholders would eat them alive.

The final elimination process brings it down to 5 highly likely possibilites.

1. Twisted Metal
2. Syphon Filter
3. Final Fantasy VII
4. Metal Gear Solid 5
5. Untitled Rockstar game