So, I was thinking of writing this story where these characters are stuck in a dangerous behavioral experiment to determine where they fit into a certain personality type in extreme conditions, but I'm not sure that I should use a actual popular testing system (like MBTI, Enneagram, ETC.)
I was wondering if there's anyway to come up with something unrelated to these tests while still making sense, I'm drawing blanks.
Maybe a uquiz? You can make your own answers and they add up in percentages to a final result, but its definitely not a thorough as Myers Briggs.
I do recommend use MBTI if you're not sure, its a long quiz but the result is detailed and it can make you think about things you never considered, like how your character would respond in a tough situation and stuff :))
A uquiz or existing system could always work (good idea!), but I also want to add some tips on making your own system.
As long as you're consistent within your world, you can absolutely create a personality test of your own. In fact, many authors and screenwriters have done just that, and have come up with their own unique personality typing systems that fit the world and characters they've created. I think it helps a lot with worldbuilding when done well.
To come up with your own personality typing system, you could start by thinking about the specific traits and behaviors you want to explore in your characters. Then, you could create a set of categories or types that capture all the possibilities of those traits and behaviors.
For example, you could create a system where people are classified based on their approach to risk-taking, their ability to handle stress, or their communication style. Or, you could create a system based on how people approach problem-solving, their level of empathy, and their tendency to be introverted or extroverted. Each of these have a bunch of different ways that people could act, and listing out all those possibilities gets you half-way to your own personality test based on them.
Once you have your test, you can also flesh it out further by thinking about how these your interact with each other, and how they might change over time (especially in times of stress or in times of calm). You'll probably see that these interactions are perfectly suited for interpersonal conflicts between your characters and/or the world around them.
Ultimately, the key is to create a system that feels organic to your story and characters, and that allows you to explore the themes and ideas you're interested in, even if it's not perfectly realistic and/or maps correctly to our real world.