Make the helghast the protagonist.
I can't speak for what they could do to improve the game, as I haven't played any entry in the series; however, I can speak for what has made me overlook the series, and that's primarily the weak protagonist. From the brief reading I have done on the backstory it seems like the helghast are arguably the good guys (albeit extreme in their measures). Not to mention they are way more interesting (there's a reason most of the advertising focuses on the image of the helghast).
From the advertising, I can't actually recall the slightest thing about the protagonist. I only played roughly 20 minutes into Killzone 2 and the overall grandness of the conflict was lost in the mediocrity of the characters. This ruined the experience for me, despite the fact that I really enjoyed the other aspects of the game (I actually like the weighted feel of weapons in games that everyone complained about initially and I feel that the Killzone series is great at encompassing a chaotic battlefield from the gameplay videos I've watched).
How do you breathe again?
