Basic Concept
My game proposal consists of an underwater magic shop. The world this takes place in was recently filled with magic, and around 5-10% of the population can be classified as witches. The witches do not have much power, but it is enough to warrant caution from the rest of society. The player character heard rumors of an evil witch living under the water, and decided to investigate as a member of the local law enforcement. Once the player reaches the shop, they are tasked with investigating the shop to learn more about the witch living down there and how the shop ended up underwater. They may even discover more about how the magic came to be in their world.
World Details
The underwater portion of the world for this game is inspired by the 1912 typhoon that hit China and the Shi Cheng reservoir, while the rest of the world can be compared to mid-to-late 20th century Europe or Hong Kong during the British rule.
The shop is designed to look like a traditional Chinese building that might be on the property of a rich or influential family. Everything near the store appears to be a part of a small farming village. Surrounding the shop is a phrase, “请保护这个家免受邪恶侵害” which means “please protect this home from evil”. Leading to the surface is a magic elevator made from glass. Surrounding it is a little dock for shoppers to park their boats.
Characters
Since it has yet to be decided if this will be 1st person or 3rd person, there may or may not be a detailed design for the player character. All that is decided is that they are a young recruit from the local law enforcement that has been assigned to investigate people who may be using their magic powers for evil.
The other character is the witch herself. She is in her late 40s and has always lived in that house. Currently, the plan is to have her be short with a muscular build similar to that of strong swimmers.
Backstory
When this witch was in her 20s, she was living mostly peacefully in her village when a huge flood caused by a surge of magic drove everyone else away. She initially had no magic just like the rest of the world, but would still carve out what she believed to be protective spells around her house. This ended up protecting her home and herself from the almost instantaneous rising of the sea levels. After this happened, she began studying everything that had changed and started making potions.
The town that was initially on a mountain became a seaside town. Most people escaped to there or drowned. It has been nearly 20 years since magic started appearing in this region of the world, and it has been used to improve parts of society while it harms others.

Game Concept Sketch

Map Concept

The area the player can access consists of the underwater magic shop and the dock connecting the surface to it. The color pallet differences are meant to display how lively underwater is compared to the surface.

Reference Images and Mood Board

Hong Kong for inspiration of the seaside town. It may not be accessible in-game, but it still will be referenced.

Shi Cheng ruins to study how underwater structures deteriorate over time.

Older Chinese architecture.

All images above this are referenced to see how low poly count games can still immerse players in the world. The games are, in order: A Short Hike, Sky: Children of the Light, Alba, and Monument Valley.

Genshin Impact has many colorful regions and its underwater regions are as magical as it can get, replicating that magical feel is important.