I mean I'm kind of using my idea, but maybe hearing about my idea will give you an idea for how to approach your own project.
Like you, I was kind of wondering what my next game was going to be like. All of my projects up to this point had been pretty safe and I had exhausted all of the "make a game in a weekend" ideas that I cared to do. While I was trying to decide what to make a friend of mine made the mistake of opening her mouth and saying she'd like to make a game. She's an artist that didn't know how to approach the software aspect, and I had already been using godot for a few months, so we decided we could form a team of sorts. She expressed an interest in making a visual novel, since it "probably wouldn't be too hard." It wasn't exactly what I wanted to program. I kind of had my heart set on something a little more interesting, like a platformer. But doing something as a team sounded cool so I got to work on figuring out how to work on that.
We needed a story, so I got to work on that. Mysteries are something that have always fascinated me, so I decided to do something with that. I knew I wanted it to be based in the the late 1800/early 1900's for the gumshoe vibes. I also knew I wanted it to include horror elements because the artist really likes horror. We kept talking and they kept insisting on making things as small as possible. I got the impression that they did not want to draw a ton of art assets. So we decided the game could take place in a single location, like an old castle in England, which would remove the need to draw many environments.
While I worked on the game and other coding projects I realized that I wasn't really sold on the game. I have faith in my writing, but it's not like visual novels are a genre I'm crazy about. It's not that hard to code and once I learned the Dialogic plugin existed most of the work was done for me. I wanted to make something more interesting, and with the artist's attention drifting more and more towards becoming a Vtuber, we talked about the game less and less. I don't begrudge anyone for changing their mind or losing interest in something. On one hand, I probably don't have an artist anymore. On the other, I can take the idea we started in any direction I want.
A few months ago I got in some game time and did some research on survival horror games. I'm not sure if you know what Blue Prince is, but it's effectively a Myst/Rogue-lite where you build an ever changing house over and over again with different floor plans to solve puzzles. The writing was excellent and while I did need a guide to complete the last few puzzles I did manage to invest 80+ hours into it. Once I experienced it, I knew I wanted to do a game like it. But I knew I couldn't do puzzles like the ones that were done in this game. Partially because they were very complex, and partially because they weren't really my cup of tea. Some of the clues are really out there.
My horror game research however brought me back to a game I had played a very long time ago. It was a clone of "Monster Maze," which puts the player in a "3d" maze where they have to avoid a T-Rex while getting to the exit. There's no sound, but you are given messages as you explore to let you know where the T-Rex is in relation to you.
"The T-Rex smells you."
"The T-Rex is coming closer."
"The T-Rex has spotted you!"
The messages were an attempt by the developer to make it less surprising so that you wouldn't get a heart attack from the T-Rex suddenly appearing on screen. Instead it causes this looming sense of dread as the T-Rex drifts closer and further away from you. Can you get away from them? Are you cornered? The T-Rex isn't scary, but it very much reinforces the idea that you're in a game of cat and mouse with this giant lizard. It's very low fidelity, and many of the missing comfort features make the game even scarier. You can't peak around corners. You have to step out into an intersection if you want to get a better look around. Your only indication of where the T-Rex is comes from some cryptic messages. You don't have a map, so you need to rely on your memory to navigate a maze that largely looks the same wherever you go.
I had my game idea.
You would play a detective. It would be a visual novel, but the castle where you look for clues/witnesses would be haunted, or perhaps stalked by the killer. So you'd have these avoid/chase sequences where you try to maneuver around the level and get to where you need to go. But what makes it kind of interesting is that our protagonist has some memory problems. He tends to forget or misremember things a lot. This is where the Blue Prince influence comes in. The map is actually created by the player. When they get to a part of the maze they haven't entered before, they will be shown some "memories" of the area and have to select one to progress. So instead of being a casual, yet tough puzzle game, it becomes more of an arcade experience. You're enemy isn't RNG or your resources, it's the thing that's following you. The more time you spend making choices, the more time your pursuer has to catch up to you.
I told the artist about it, and it did seem to make them more interested in the project again. Though I'll consider them interested in an official capacity if they ever draw something for the game again. Right now they're pretty busy with commissions. But more important than how they feel about the game is how I feel about it. At this point I'm willing to learn how to sprite better or draw for real if I have to.
I guess all this rambling is to say that when in doubt, go to your roots. It's important to realize that if this is your first time doing this it's going to suck or be kinda jank. But it'll turn out a lot better if it's something you're interested in or already have experience with. This game idea fits pretty squarely in my wheelhouse. I ran a murder mystery/trial RP for 5 years. I pretty much exclusively played horror games in my teenage years. There are going to be setbacks and it likely won't be as good as I hope it'll be, but it could be fun to play. At the very least, I'll learn something.
Look closely at your knowledge base. Consider basing a game off something you know well and merging something you're interested in with it. I can't tell you the number of devblogs I've followed where someone's cool game idea gets abandoned because the creator loses faith in it. They all want to make something new and revolutionary as their first game. Meanwhile some dude decides to make "A Game About Feeding a Black Hole" and releases it on steam about a month ago. It's grossed over half a million and everyone's rushing to make clones of his game. But when I look at it, I just see something akin to a low effort flash game I played 20 years ago.
For you in particular though, and I don't pretend to know you that well, you've participated in a lot of magical girl and Digimon RPs. Maybe find some way to fuse those two things? A creature collection game where you collect magical girls? Magical girls that do battle against each other and monsters with their own monsters?
