One totally optional, but (in my opinion) totally fun aspect of world building is making up fictional games for your world. Like made-up holidays and festivals, games unique to your world can give it a deeper feel and provide an endless source of subplots, conflicts, and climactic settings.
For a lot of fictional games, the rules don't actually matter. Nobody knows how to play
that chess game R2-D2 plays against Chewbacca, but the scene gives the world a deeper feel and gives us a taste of Chewbacca's character (also Han's and C-3P0's).
Avatar: the Last Airbender frequently uses a game called
Pai Sho to reveal things about one of the characters, but the rules are never explained.
But sometimes you want more than that. A critical event might turn on the outcome of a bet, like in
Pirates 2 or
Phantom Menace. You might have climactic events that center on the playing field, like Harry Potter's Quidditch. Or your entire plot might center on a game, like
Ender's Battle Room. In these cases, the reader needs to understand and care about what's going on. They need to know the rules, which means there need to BE rules.
The easiest way to make a fictional game is to take a real-world game and change it slightly. Take chess and give the pieces fantasy names. Take soccer* and give it two goals instead of one, or play with three teams at once on a circular field. But whatever you do -- whether you vary a real game or invent one of your own -- it needs to be a game that, for the most part, would make sense in the real world.
Here's where Quidditch fails. The made-up game starts okay: basically basketball with broomsticks, three goals per team instead of one, extra balls that hurt/distract the players, and a snitch to determine the end of the game. None of these variations break the game, and they all make it more interesting. If we had flying broomsticks and semi-sentient balls, this is a game we could play in the real world.
The problem is the point value of the snitch. Every goal in Quidditch is worth 10 points, but whoever grabs the snitch simultaneously ends the game and earns 150 points -- 15 goals. The overall effect is that regular goals don't matter. Ever. Unless the score reaches 15-0,** the rest of the game has exactly the same tension as if both teams just sat around and waited for the snitch to show up (which, really, why don't they?).
The only reason we don't notice is because the protagonist is the one who gets the snitch. Can you imagine if Harry was the one making meaningless goals, while some minor character caught the snitch and won the game? We also don't notice because usually something else is going on during the match -- like someone's trying to kill Harry or something -- so we don't actually have to pay attention to the match. But to me, all the wizards who go crazy over every goal seem silly and short-sighted.
So by all means, include made-up games in your world. But give them some thought. They don't have to win
Game of the Year or anything, but they should at least make real-world sense.
Though I guess if you really are writing the next Harry Potter, it doesn't matter.
UPDATE: As I mention in the comments, I do like Harry Potter. A lot. It has it's flaws, but there's a reason I own all seven.
* A term I use, not because it's correct, but because it's the least ambiguous. They call it football in Thailand too.
** Which is ridiculous. When was the last time you saw a soccer team up 15-0? Or an American football game at 105 to nothing? Unless you were watching Big Leagues Beat Up on Tiny Tots Day, these scores just don't happen. Not at a professional level anyway.
