I'm currently working on the initial treatment document of the game called Critters. Here is the game overview. All mistakes clearly are there to test you, so comment if you see any.
The main idea of the game is using creatures instead of spells in action/RPG. The character simply mounts each new creature after choosing it from the 'spellbook' or in our case from the bestiary. Each critter has its own abilities which give player a wide range of in-game actions from rooting or stunning the opponent to running faster and jumping higher.
Different critters and combinations of critters (one critter after another) are needed for different missions, which is exactly why the player must try hard to gather as many critters as he or she can. To add a critter to his or her bestiary, the player has to accomplish a minigame, which is unique for each critter.
The genre of the game is action/adventure with some RPG elements including perk system, spellbar (with critters on it), quest log and quests themselves. Each critter has two abilities (one for each mouse button). Moreover, these abilities can combine forming another (probably more powerful) interaction. To make a combo the player has to press mouse buttons in a specific way. On a gamepad this kind of controls can be realized via triggers.
The player can tame only those critters which suite his character's class. In whole there are four character classes in the game: the demono, the dru, the hypno and the warri. Demono specializes in critters with major magic abilities such as fireballs, lightnings, freezing and earthquakes. Dru's critters can control nature: they are various fairies, unicorns, dryads as well as everything else dealing with trees, grasses and forests. Hypno's specialization are creatures which can stun, hypnotize, confuse or even mind control. And, finally, warri controls the most strong, savage and fierce critters.
All critters in the game are based on the mythology of different parts of the world, so players will be able to recognize most of them. The game setting is a bit of mythology combined with dark fairy-tale fantasy, as such of Grimm Brothers' fairy-tales. In this world gremlins wage a war against humans, who have established a steam-punk industrial society, which is expanding rapidly, leaving the creatures of the wilds less and less country to live. Each critter can and will talk to the player, making jokes of him and choices he makes, as well as giving advice.
Taming critters is conducted the following way. The player has a journal which contains descriptions of all critters, their behaviors and habitats. Moreover, he has a world map which becomes detailed in course of world exploration. To tame a critter the player has to come to a location described in the journal and perform a specific rite-minigame there.
The design of the game world is done in a way that forces the player to encounter a specific number of action on his way to the next critter or quest objective, however, the player has to choose a route himself.
The game has a coop mode which will allow up to four players to be in the game world at the same time. Each player has to pick one character class. The game world is modified based on the number of players in it. The more players—the tougher are enemies. But the minigames are modified as well, making it possible to complete each of them in the coop mode as well as in single player mode.
The competitive multiplayer mode includes both minigames and arena in team against team, team against AI, single player against another single player. Each multiplayer mode has its own leaderboard and achievements system.