If you're thinking of selling a game, there are actually some pretty good excuses you can put in your game to charge more money for it. This article will cover some important ones.

Shooters: You should definitely put in a level editor. Once the player has completed the game, you might want to allow them to play the same levels using a different set of weapons. Allow the player to play levels that s/he has already completed. Include some separate levels that the player can act as a god over (allowing the player to make water pistols shoot napalm, or allowing BB guns to destroy a skyscraper in one hit).

Platformers: Level editors are a must (as in everything). Give them a god mode (again). For some games, you might want to include modes that penalize the character for staying still (things attacking them every 5 seconds, an invisible force pushing them forward.)

Arcade Games: Arcade games are simple, so there's not much to say. Give bonus levels, control everything levels, etc...

Strategy: Definitely include a world editor. If you can, you should allow multiplayer online support, or, if the game is turn-based, hotseat multiplayer.

RPG's: RPG's are pretty linear, so there's not much you can do. Try allowing the player to create their own adventures using your characters and monsters.

Sports games: If your game uses different preset players, give them a "Create a Player" tool. Give them a certain number of points and allow them to use those points to give the player attributes. Allow them to change the setting. It might be fun to play tennis on the moon. And while you're at it, allow them to change factors like gravity!

Racing games: No racing game is complete without a track editor. There are two types of modes in a racing game, story (progressive racing) and loose (racing on tracks with no attached storyline or progressiveness). Your game should most definitely include both. For your story mode, try including things such as unlocking tracks to be used in loose mode, upgrading your vehicle Star Wars Racer style, etcetera. Allow a “Create a Vehicle” option in much the same way as in the sports-game’s “Create a Player”.

Construction and Management Simulations: There are two modes to this type of game. They are management and god (building a house in a millisecond in The Sims). Allow both. Include direct feedback (eg: “Business is booming, and the stocks are rising. Good job!” or “Aaaaah, curse you! Our value is dropping like crazy, and it’s all because of you! We’re thinking of overthrowing you, but because this is a game and you’re the player, we can’t. Down with you!”). This will allow players to use the god mode to torture their employees/citizens/customers/workers/whatever. You can’t charge much extra for this, and there isn’t really much else (or anything) you can do.

Adventure Games: Like RPG’s, adventures are very linear. However, you can allow the players to create their own puzzles using your objects. They should be able to do anything with any object, meaning they could take a gun, point it at the window, and be asked if they want to “Perform striptease for Fred”.

Puzzle Games: Puzzle creators are a must. You might want to include a two player mode, where two players take turns making moves to complete the puzzle.

If you’re planning on selling a game, hopefully this helped you create an excuse to rip your customers off even more.