Having BNS back in practice mode has helped a lot, but there are a couple more things that could probably be done very easily and would help make practice mode a better part of the game:
1. Ability to manually specify a short passage and play it over and over again like in the drum trainer. If the chokepoint that keeps killing you is only 5-10% of the whole section of the song that contains it, it would take 10-20 hours to practice the part you really need to for one hour. As a result, practice mode doesn't really help that much if your goal is to become able to FC a song rather than just fight your way through it. Usually I just have to shrug and hope that after another year or two I'll have built up the instincts and accuracy I need to hit that passage without having to focus on it.
2. Even slower speeds than 50% before. I don't think 20% speed would be a bad idea, especially when people are trying to learn pro guitar or keys. That was always the way I learned difficult passages in real life (on saxophone ) was by playing them very, very slow until my fingers had finally learned the dance steps, then playing it gradually faster once I could count on my fingers to execute the correct pattern. It would also be good helping you retrain yourself to break bad habits and for diagnosing the problem when whatever goes wrong happens so fast (even at moderately reduced speed) that it's hard to determine precisely where the combo broke and what the problem was.
Some of us out here still can't just immediately FC everything that's thrown at us and would benefit a lot from these.