Yeah, trane's design works really well for practical stuff like learning an instrument. It's probably good for language learning, too: perhaps a series of books or texts, each of which builds on the language skills learned from the previous…
It's a good idea to limit the scope of your projects. Jack of all trades is a master of none, and the fact trane exists (and is GPL'd) makes it just that bit easier for me to solve the other problem.
I guess you are right. The textbooks definitely have dependencies, but that seems to be the very problem that you have with them. Unfortunately, I think other than different curriculums for each way to approach the subject, this is beyond the current scope of Trane. I made this thinking of music at first, which has a more rigid way of approaching the subject that tends to work for a wide set of students. There's no person on earth that will be comfortable studying highly chromatic music without mastering harmonically easier material first.