So on my Android using the Next Cloud app, I generally like to write and read documents using markdown, because .docx extension does not open on mobile. Of course, it would be nice if that started working, since you can already leave comments on .docx files (at least on desktop), But I thought it might be easier to add the ability to comment on markdown docs. I realize maybe a little tricky with rendering, making sure the comments point to the right place in the page, But I wanted to see if this might be on People’s radar for the development roadmap.
I don’t think there will be such a change as AFAIK comments are not part of markdown format - but you may use some format you specially consider “comment” within your notes.
And definitely you can open Office files using “Nextcloud Office” which integrates Collabora suite into Nextcloud UI.