However, I need to curb my enthusiasm and not implement more features than I strictly need right now.
One thing I would like to have (that you don't find in schema languages generally) is the ability to define enums as a query on another document. For example, a career lists a number of skills, mapped to the maximum progression possible for that skill in that career. I could define an enum in the career-schema.treeml file defining the expected skills, but that is fragile, too much work, and beyond my mental ability to keep straight. Instead I would like to define the enum as a reference to the careers.treeml file:
enum : flow, skills, args, "skills.treeml", "skill/id"
Then my career schema is always in synch with the skills list! I'd apply the same thing to the list of career possessions, etc. The above example is not quite well formed as the last two arguments in the value list are not tokens. Also the "filename" should be considered a logical resource name and not a system dependent file name.