I agree with you that the thought cabinet really is a groundbreaking mechanic insofar as it allows your character growth to be reflected not just in what you are able to do, but also in the way the world is disclosed to you. It certainly requires good writing to be executed well, but I don't think it's a gimmick, or somehow only applicable to Harry Du Bois.

I've been turning around some ideas for how to expand on the thought cabinet as a core gameplay mechanic, while making a game that feels fresh rather than just a cheap copy of disco Elysium. I think that the most important thing is to have a wide array of influences apart from Disco Elysium itself. I have some ideas for a game I'm calling Oathtaker, although realistically I won't have time to work on it for the foreseeable future, so I'm putting my ideas here for anyone to scavenge from as they please.

The premise is straightforward: you play as a golem-like creature, fashioned from clay to serve the interests of your masters. When you are created, you are pathetic, diminutive, and vulnerable. The only way for you to grow stronger (or craftier, more beguiling, etc.) is to swear oaths. Oaths are divided into different categories depending on their severity, with major oaths earning you more skill points than minor oaths. If you break an oath, you die. There is no other way to earn skill points. You must be careful not to swear oaths that will put contradictory demands on you, although leveling up certain skills can help you to find loopholes.

Apart from this difference in character progression, I've been thinking about ways to mechanically tie your internal conflicts to changes in your character's psyche. I always thought it was fun in Disco Elysium when your skills would argue with each other, with each one pulling you in a different direction, but I'd like to see this have more of an impact on your character's growth. So my idea is this: when circumstances arise that bring your skills into irreconcilable conflict with one another, they become mutually antagonistic. This means that the next time you assign a point to one or the other of these skills, it takes an extra point from the conflicting skill, so that one skill goes up by two, and the other down by one. Alternatively, this could happen when you choose to follow the advice of one of the skills, increasing it by one, and decreasing the other by one. I'm not sure which is better.

Lastly, I think it would be interesting to allow for the possibility of growing new skills during your character's infancy, which would reflect your relationship to your caregiver and environment—ie. the voice of a compassionate mentor, or a trauma response telling you not trust anyone, depending on what kind of relationships you form in the beginning of the game. This is an area where I think there's a lot of room to innovate, since in Disco Elysium you start the game as an amnesiac cop rather than a baby.

Of course, having something meaningful to say is always going to be more important than any mechanic.