You can use Google Docs during early brainstorming phase. It is a good tool to dump idea in early stage and have a collabarative discussion with each of them.
- When a document is shared with someone you can specifically select/highlight portion of text and mark a comment on that.
- To mark a comment use shortcut "Cmd+Alt+m" on Mac or "Ctrl+Alt+m" on windows.
- You can mention specific user by using @ symbol.
- When you mention the user in comment, the mentioned user gets notification.
- Replying to that notification through email will post your message as a reply to original comment.
- Got a rough diagram - Google docs allow you to insert one inline from "Insert > Drawing" menu.