Will Substack be strictly a written newsletter platform or will it allow a hybrid approach including picture, audio, video, comments, etc. ?
You can already add pictures. We are testing comments for paid subscribers on a couple of publications (you heard it here first!) And that other stuff is definitely interesting.
What will not change is that the experience for the reader will stay really simple - Sign up, and everything you need shows up in your email. Also the model will stay focused on letting readers pay writers directly for high quality content, because that's the magic.