Option to set a second passcode that either opens a limited set of chats, opens a different account, or destroys one of the connected accounts completely when entered ("double bottom").
Could be used by political activists in opressive countries when they are required to unlock their chats.
If such an emergency passcode is set, the app won't be able to use your fingerprints or other biometric data to unlock chats. As a result, the absense of biometric unlock could be used a clear indicator that a person has something significant to hide – and it becomes impossible to prove this is not the case.
This would provide a false sense of security and might actually make the situation worse for an activist's or an innocent bystander.
Related suggestion: Separate passcode lock for specific chats or Archive