Anything that converts a guest to a user due to the offering / features

Hi, if you don’t mind sharing; What do you feel is the conversion feature from kindle? I am developing on a platform with novels and want to develop user centric features; I have feature where you get assisted highlighting of words with auto scroll so you can “follow the word” and read.

I have the architectural design in place to incorporate tts for example but would love feedback from the reading communities