Killzone 3 will sell 1 million more copies than its previous installment. Kz2 is currently at 2.5m so my prediction is atleast 3.5 LTD for KZ3.
Here are my reasons for thinking there will be a lot of growth for this franchise.
1. User base. It doesnt always make a difference, but it will for Killzone. KZ2 came out pre-Slim when PS3 only had 22-25 million users (cant remember the exact number). Ps3 is now at 45million. Possibly double the amount of when KZ2 released. When KZ2 was released, PS3 was still $400, PSN wasn't as good as it is now, and the system as a whole had a bad reputation. Since then, the PS3 was rebranded and rejuvenated, the price dropped, and PSN has been greatly improved.
2. Brand Name and Momentum. When KZ2 came out, the Killzone franchises and Guerilla Games were still somewhat unproven. They were an enigma. Now 2 years later, after many good reviews and word of mouth, Killzone is a legitimate franchise in the gaming world and Guerrilla has established themselves as a potential powerhouse. Mopmentum wise, most big exclusives that have released after the PS3 Slim have sold above expectations (Heavy Rain and Uncharted 2, are good examples of that)
3. Marketing. Simply put, Killzone 2 had almost zero marketing. Fast forward to today, PS3 commercials are all over the place. I'm actually getting sick of seeing Kevin Butlers face so much in the commercials. Sony has definitely stepped up there game here so its safe to say KZ3 will receive much better marketing than KZ2 did. Sony is already advertising Killzone 3 a full 2 months before it's release date:
http://www.youtube.com/watch?v=aOSnR9I1src
4. This might be the biggest reason I think KZ3 will sell much more than KZ2 did.... Back when KZ2 came out, FPS games in general were not all that popular on PS3. CoD4 had good sales, and WaW sales were decent. But they were nothing compared to MW2 and Black Ops sales. It's obvious that since the CALL OF DUTY BOOM has happened, PS3 gamers are much more open to playing FPS games now.
5. MOVE and 3D support. Sony has every reason in the world to push this game as much as possible. MOVE users that normally wouldnt play KZ will try it out because they want a reason to use their new motion controller. I also imagine KZ3 will be a popular display/sales strategy in the big-box stores that want to push 3D TV's.
6. More user friendly controls. Judging from the Beta, this came is going to appeal to a bigger audience. Faster, smoother controls. More online gameplay modes. Better cinematics in the campaign. Split Screen now added. These are all big selling points to the mainstream FPS crowd, and should also ensure improved reviews for Killzone 3.
