Provide Text to Speech out of the box, a test case could check if Foliate can read a book with it installed.
PicoTTS is good candidate, it only takes a few megabytes and sounds probably better than other open-source engines. The bad part is it lacks many languages.
Maybe one with GUI would be better even at quality cost so that users would be more aware of it.
MyCroft uses Festival iirc, that may be a good one, too.