Right now, not much can be said about the sequel to a Playstation's best seller except that it will be very different from the original. Instead of four playable characters, now you only control one of the four, Hawk. The game will revolve more around weapon and gun combat than hand to hand. As in the first game, there will be a lot of interaction with the background and scenery. Also, the game will now have a much darker feel, not unlike Metal Gear Solid. The 3D engine has also been redone.
The game will be much much longer too. There will be roughly twenty levels and there have been rumors that one of the levels is as long as the first game! They have also said the levels will be long and complex, just like Tomb Raider. Although there is only one playable character, there is supposed to be a lot more characters to give the story depth
The controls have also been tweaked. It is easier to kick and move around. The enemies will also be much smarter. Instead of standing in there little spots waiting for you to beat them to death, they will follow you around and make strategies to kill you.
The first game seemed to get monstrous after a while but Eidos says that will change with this game. I think Eidos games are a little boring and devoid of storyline, but we'll just have to see how this comes out.