Firstly i think a change in location would be great, where the second and third where on helgan this one appears to be on vekta which allows the game producers to have much more color and variety in the design and graphics of the game. We have already seen there is a segregated society with the helgast on one side of a wall and the vektans on the other so this allows then to have a darker side with the helgans and a more colorful theme with the vektan side.
One other feature i would love to see is a campain that focuses on both sides of the war, i want to see it from the helgast point of view aswell as the vektans, this opens a world of possibility as we have seen with the gameplay from the ps4 announcement when a helgast soldier is disguised as a security guard. This sort of thing can create great gameplay moments like being undercover on the other side of the wall or planting a bomb whilst trying to remain undetected to take out a high ranking soldier of the enemy.
Also i want to see more vehicle sections such as the mech in killzone 2 and the jetpack in killzone 3, these added variety to the linear gameplay whilst the jet pack gave you more ways to tackle the mission. I would love to pilot one of the fighters that is being flown in the gameplay trailer, gunning down helgast whilst dodging in and out of skyscrappers and missiles sounds good to me.
One of the biggest expectations for me is the implementation of a co-op campain. I don't think it will be too difficult for them to put this in there as the story in killzone 2 and 3 already had a big emphasis on your squad and developed your squad mates characters through the cut scenes and dialogue, so seeing your friend take control of one of your squad seems like a fantastic idea to me.
And finally i just want killzone to be as different as possible so it isnt just a call of duty or battlefield clone with boring linear corridor shooting. Also it is a ps4 launch title so to encourage sales it is going to have to be special and stand out from the crowd to make people fork out another 400£ for a new system.