I'll further illustrate the importance meshing game design and mechanics with two contrasting fantasy/sci-fi examples.
The Obvious Example
Let's say you're designing a children's game with a Hitchhiker's Guide to the Universe kind of offbeat goofiness.
In this game, set in London, you play "Tyrone Camden McMead, ever so posh yet relaxed in his tweed." He's a man who hates cats. After Tyrone punishes the Grand Cat (the cat who rules all other cats), the Grand Cat turns him (you) into a cat. At this point, the game begins.
Your goal is to stop a group of adorable wiener dogs from spreading through the city. You know they're really the forward flank of an invading alien force.
This group of wiener dogs are spreading a secret odor that makes Brits love wiener dogs so much that they'll acquire more and more of them until the entire country is literally overrun by millions of wiener dogs.
The streets become a mass of writhing wiener dogs. In certain sectors of the city, wiener dogs are piled high in every house and shop. There's no more food for people to eat, and they get crushed below the towering carpet of wiener dogs. Big Ben is silenced, and becomes Big Has-Been. Soon the aliens will swoop in. Britain will be theirs for the taking.
What are the mechanics we'll use here? Obviously, because you play a cat, they involve such things as leaping, scratching, squeezing through tight spaces, landing on your feet, attracting by purring, repelling by hissing, and many other skills my college girlfriend had mastered.
The moral of the story: Because you're a cat, the mechanics are pretty obvious, and they'll fit in perfectly with the story.
The Not-So-Obvious Example
Now let's take a more challenging example.
Let's say that there's a game where you're an explorer on Mars, and you find that the planet is inhabited by ghosts human ghosts. In fact, you learn that an evil, alien race brings people's souls here between lives. (In this game, reincarnation is real.) Once the human souls are here, the aliens erase their memories and implant new identities and purposes into the souls, and then send them back to Earth.
The goal of the aliens is to ensure that Earthlings are so alienated from their innate personalities, with wrong goals, altered identities, and needless worries, that they will undermine themselves and their world. It's all a way to keep them screwed up, ineffective, and self-destructive. Earth is thus a prison planet that needs no guards.
In the game, at first you need to deal with (fight) some tormented ghosts. Later, you'll need to take on the aliens. Finally, you'll need to go back to Earth and deal with some highly placed political leaders who are in collusion with the aliens.
What kind of mechanics suggest themselves to mesh with the game? It's not nearly so clear as with the cat example.
There are two approaches, and I'd probably apply them both.
One would be to make a list of necessary almost obvious mechanics and weapons, such as:
I'd try to make these items as imaginative, fun, and interesting to use as I could. Of course, all this is the obvious route any game designer would take.
The second approach would be more of a thematic one. This is the approach I suggested for Final Fantasy X.
Just as Final Fantasy X revolved around dreams, this game revolves around life, death, and identity. Therefore, I'd want some gameplay mechanics or weapons that reflect these themes.
For instance, I might create some kind of ray or device (the "Lazarus Ray") that can bring dead or dying people, animals, and plants back to life, although perhaps I could only use it on rare occasions. Or maybe the device is powered by my own life force I get drained of life as I give life to others.
As the game is about people passing into a ghost state, I'd want some kind of method whereby I could temporarily die, or at least turn myself into a ghost. Maybe, once I turn into a ghost, I have new abilities, such as moving through walls, spying on the aliens without easily being seen, or changing into a frightening appearance.
I would need to use these mechanics bringing things to life and being able to become a ghost for periods of time to win the game.
Because the aliens reprogram the identities of the ghosts before they reincarnate, then I'd want some way to awaken them from this programming.
It could be a device or form of gameplay such as:
In short, I'd want mechanics and gameplay that related to the themes of life and death, as well as identity.