Even after 3 years since its release I find Killzone enjoyable to play.
Maybe we will have to wait to see what Killzone 2 offers, hopefully something like:
Improved Offline Story
Customisation of Weapons/Clothing Offline and Online
Extended Clan/Game/Online features, Game being Types and Sizes
Vehicles, not Helicopters though
Can't wait for Killzone 2, if it delivers it will probably be set as one of the best games made, considering how well Killzone has done even though it lacked in areas and could have been perfected more.