Along the same lines as this feature request:
A built-in text-to-speech feature (enabled on the SPEAKER's side, not the listener's side) would help communication when speaking out loud is not possible (for example, those who are non-verbal) or desirable (playing with people sleeping nearby). There is some third party software that does this, but integrating it into the client makes it easier to setup.
By default the feature should limit the size of text it will speak out, and throttle number of text to speech messages per second, to prevent abuse. Messages between friends would not necessarily be subject to these limitations. User would toggle text to speech in their chat UI. This should be OFF by default, but toggled on not by the people listening but by the person speaking - in other words, it's an accessibility feature people would use instead of using their voice and thus any text to speech messages would be heard in any situation where voice/mic would be heard.
Of course it would be subject to blocking/etc...