I will buy it if:
1) The enviroments get some more color in them, the graphics in Killzone 2 were amazing but by the time I got to the end I was sick of seeing the same dull grey textures.
2) The single-player campaign is longer, not that I thought KZ2 was short.
3) You get to kill Rico in a friendily fire accident at that start of the game
4) The controls were lighter (I know it's meant to simulate carrying all the equipment), it was just annoying that it took so long to turn and shoot someone that is behind you
5) There are bigger online maps, and more players able to play
6) A larger variety of weapons
....and finally....make the graphics better(if that's possible) ;)