Exactly what Pristine20 and BHR-3 said, that happens and the game would be perfect

Griffball and Zombies!

Come on, Halo 3 has Griffball, and Left 4 Dead and World at War has zombies. One can't call Killzone king unless it least has zombies and Griffball!

ROFL!!

OT: Rubber Duck Grenades

And More water.

A couple of things:

Kill off the annoying Rico instead of characters you actually like.

Keep the controls essentially like they are, but make some tweaks (the shotgun moves much faster than the rifle, but the revolver moves exactly the same as the rifle...wtf), and overall improve it a bit.

Definitely incorporate the backstory more, and improve the story overall.  Hire some professional writers or something and give them more control if need be.  Iain Howe and the guys came up with a great universe, and their writing in the histories is excellent, but they could've conveyed KZ2's story so much better, and filled in the gaps more.  Sometimes I had no idea why what was going on was going on.

Improve the online system a bit.  Allow us to invite friends that are online to our games so we don't have to message everybody via PSN when we create a new one.  Maybe make it more like LBP, where you can just bring up the start menu and start shooting of invites (or join another persons game).

And some other stuff.

1. Rico's death
2. Superior graphics, that would be sweet
3. More electric gun use!

WIN

More new things for MP.
And probably more weapons. :)

A free beer and back rub.