Metroidvania

Core Concepts

 * Boomerang is the main tool.
 * Focus on being able to use the boomerang in a myriad of different ways.
 * It can be thrown, but also swung like a melee weapon akin to Kirby's Cutter ability.
 * Basic story, but lots of background lore.
 * Fast travel via a Warp function of the boomerang.
 * Metroid-esque saving areas.

Story

 * Protagonist stumbles upon a ruin that they were unaware of existing prior, and heads inside.
 * Finds the boomerang, which can speak to them and has a mind of its own.
 * The boomerang tells the protagonist that the ruin is a gateway between worlds, so to speak, and that if they work together to gather five (insert Macguffin name), they can unlock the true treasure of the ruin.
 * Over time, the player begins to see the presence of the main antagonist, with a connection being drawn in the similarity of the boomerang and the weapons used by the villain's minions.

Boomerang Abilities

 * Affixes are permanent buffs to the boomerang, while Runes are active abilities you can apply to the boomerang. Some Runes use up Magic Energy, which slowly regenerates.

Affixes

 * Double Shot: The boomerang creates an afterimage that flies next to it.
 * Triple Shot: The boomerang creates two afterimages that appear above and below it with different angles.
 * Piercing Shot: The boomerang can pierce through enemies.
 * Delivery: The boomerang can pull items towards you.
 * Aquatic: The boomerang now functions normally while underwater. Used to gate off access to a specific area until later on.

Runes

 * Hook: The boomerang pulls you up to where it hits, like a grappling hook.
 * Far Sight: The boomerang grants you visibility of where it flies.
 * Manual Control: The boomerang can have its flight controlled manually.
 * Scanner: The boomerang will emit a pulse of energy at any solid surface it hits that reveals lore about that object along with revealing anything hidden.
 * Gust: The boomerang creates a tornado around itself that can push objects and enemies.
 * Flame: The boomerang is lit on fire and can burn things it hits.
 * Stalagmite: The boomerang creates pillars of stone beneath the path of its flight.
 * Magnet: The boomerang attracts metallic objects to it.

Areas

 * The Descent
 * Main halls of the ruins, more or less a vertical shaft.
 * Guts of the Unknown
 * The insides of some kind of beast, with monsters that resemble cells and body parts.
 * Treetop Wastes
 * A giant jungle with trees that aren't always facing upwards as well as floating islands.
 * Aquatic Depths
 * Cavernous areas filled with water and dangerous fishlike enemies.
 * Frozen Caldera
 * Volcanic area surrounded by ice, aka a mix of hot and cold.
 * Zeroone Temple
 * A giant temple that's technological in nature, with broken-down machines, some of which act as enemies and are infused with the villain's magic.
 * Vault of the Sosak
 * The ancient prison that holds the main villain, and where the final battle is. The area leading up to the boss room is meant to test your abilities.