Indeed, you can't really prove why a particular franchise gets popular (i.e. good sales). You could look at the overall picture (i.e. all of the popular games, all FPS, etc.) and look for correlation, though. This way you get common trends. However, this doesn't mean that a game based on that findings would be popular. After all correlation is not causation
This... And more...
There is no sure thing. If you are developing games, you KNOW you'll have flops. Flops that will lose you money.
In the process, you hope you guessed right on a project to OK. More importantly you hope one of the projects you OK will earn "oodles" of money. Enough money to cover the flops and continue funding for developement of new titles.
The problem is, good ideas do not always equal games that people want to buy and play.
Good ideas sometimes don't translate well into good gameplay.
The problem is, when you decide to fund a game, all you have are a bunch of ideas... And then you follow through and hope it's a hit, but know you'll get some flops.
Game critics can't tell you what will sell or not sell. It's quite possible that while they may like a game, that the majority of people just don't share that view.
For example, I personally HATE open world "sandbox" type games in general. But those games are far far more likely to garner good ratings from game critics than a game on rails.
Likewise, people seem to give higher ratings to games that are somehow new or innovative, even if what people might want is just more of the same of something they already enjoy. I would have been happy for example if Fallout 2 featured the exact same gameplay as Fallout 1 except with a different storyline. And would have been even happier if Fallout 3 had the exact same gameplay as Fallout 1, except with a different storyline.
Part of the Halo/COD series success is that they don't stray too far from what people like about the Halo/COD series. They're all basically the same with better graphics/different stories.
About the closest thing you can get to a sure thing in game developement is a good developer with a proven track record. And even that isn't a sure thing.
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For KZ2, I like the fact that they wanted to inject some realism into the game with weighted weapons. I personally thought that was absolutely brilliant.
A pistol SHOULD be faster to swing around and aim at someone close to you than a rifle. Just like real life. There's a reason why you want smaller lighter weapons in close range combat (especially in confined spaces) over larger more powerful and longer range weapons.
However, anyone who thought something like that would garner anything but critical praise from the media/gaming elite were smoking crack. Success for critical praise, check. Success for massive appeal and massive sales? Nope, sorry, not going to happen.
That's where you can run into disparities between what's a good "idea," what will probably gain "critical praise," but won't get you stellar sales.
On it's own probably not a killer, especially if the game had a superb storyline... Or superb online with co-op, well balanced versus modes, easy matchmaking, etc...
But in the end, it's a technically superb graphics showcase. With some great features and an attempt to bring something different to FPS games...
And when all is said and done, it just might be a case that most people just want more of the same rather than something different...
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The flip side of that, of course, is that while too much innovation can kill your game. It's also innovation that might bring a whole new genre with you at it's leading wave reaping the rewards...
The problem is... There's just no way to know that until you take the gamble and try it...
Regards,
SB