This is Unity, and that wouldn't be out of character but actually no... This is still the normal Realmyst ME running on its usual engine (an older version of Unity). Although I think Cyan did improve the lighting a bit since its initial release, as well as a few models, so there is that.
But as you can guess from the white debug text, glitched tower visuals and 3d text mesh hovering around, I finally found a way to modify the existing game
I can't modify the data archives themselves, but I can inject custom code into the game at runtime. This allows me to mess with the scenes, load custom 3d models and override/extend some of RMME's behavior.