Aside from the typical encryption worries here are some features I would love to see being offered on a alternative messenger app
-ability to give out your alias or username to people to reach you on whatsapp instead of a phone number, something you can change later after banning the contact.
-transcribe voice messages to enable search among them.
-ability to hide the fact that i have listened to their voice message.
-ability to hide the fact that i am on whatsapp network and receive 'add to contact' requests from people where i can accept or reject without them knowing i actually rejected.
-hide the 'online' in conversation window when im reading the message.
-save my chat history in the cloud so that i dont lose everything when i move from iOS to Android or vise versa.
-ability to save incoming and outgoing media attachments to the cloud for access later
-full desktop support for video and voice calls
-ability to record the calls, and have their transcribed texts easily searchable.
-ability to ignore certain kind of messages from certain people such as 'allow text messages only, no calls, no pictures, no videos.'
etc.