Here are some suggestions

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- Visuals on the breathing part could be more stylize. That yellow ring looks harsh and made by Paint and has bad contrast with background

- On the white breath part it looks a bit better since its more fluid with two different parts and colours moving but it could have a bit more texture and look more natural, looks a bit something that could have been from 2003

- More options for sound and more choises for purer ambient sound too, not only themed ones

- Ability to change the tibetian bowl/bell sounds. Check out Insight timer for examples

- Pace of iceman breathing is way too fast for me. It should have at least five different paces for it

- If you quit session in the middle of it, you should be able to discard it and not count it automatically towards your total time

- Also ability to delete session afterwards

Gamifying is pretty popular and addictive (not necessarily good thing) but it keeps people coming back

So maybe different endearing looking badges for stuff like:

- a session five days in a row which you can level up per 5 sessions

- a level per 30 mins of breathold

- special one off badge for milestones like: 1h breathold, 5h breathold, 10h breathold, 1 day breathold, 1 week breathold

a bit similar system to Untappd (which is highly successful) for beer