Honestly? For it to be a massive hit, they would have to dumb it down. Make it less subtle and political. Most people can't deal with a conflict where both sides are corrupt, greedy, and ruthless. They want an ACTION HERO(!), on the side of RAINBOWS AND JUSTICE, who is out there shooting bad guys and aliens--especially aliens because they're not human and are thus mindless, evil scum. That would make me sad, as the standard "Starship Trooper" fare leaves me cold, but that's what makes a hit.
Killzone also suffers form what I call "Final Fantasy Syndrome" where previous installments have fragmented the fanbase by being different from one another. You have people who loved KZ2 style and others who hate KZ2 style and neither one will be happy with compromise. There are a LOT of gamers out there who are unable to judge a game by its own merits and instead constantly compare it to previous games.
I don't want Killzone to end but, honestly, to appeal to the mass market, they might need to try something more simplistic, with a marketable "hero" and clearly defined "good" and "bad" guys. As a person who reads a lot and loves complex universes, that makes me very unhappy to say, but that's where the money is at. Generic sells.