Congratulations, and best of luck.
Personally! What I would like to see:
1. Publishing system to major platforms (Medium, Tumblr, Wordpress,...)
3. Publish Markdown to any platform
4. A service to capture input from any software to write in Writeroom, for example compose email in Writeroom.
5. Media embedding
7. Cloud sync
8. Integration with Evernote
9. Grouping to build stacks of documents.
10. Export to all major formats (pages, docx, PDF, ePub, Mobi)
Those are some ideas I have on mind...