There are a few features it's like to see for fasting:
1) Auto detection - if I do not enter food for some period of time it would be cool for cronometer to have the option to automatically start a fast for me. Just one less manual step to tracking fasting, my fasting can be sporadic at times and it can be motivational to realize how far I am already in a fast.
2) Fasting planning - Ability to plan out a week in advance fasting hours. Just easier to manage/balance fasting and feasting if you can visually see it in a plan.
3) Fasting menu item on main front page