A simple start might be to identify in the Dave Heiland document the missing items and see where new features might be best placed.
Calling all could could be useful, here's mine (really not exhaustive; bold = new items):
Cover
Title Page
Copyright
Introduction
What’s New
Changes in 0.9 include:
About Sigil
About EPUB
Installation
Features
User Interface
Preferences
Opening and Saving Files
Book View
Code View
Preview
Book Browser
Metadata
Add Cover
Table of Contents
Validation
Spellcheck
Images, Video, Audio
Special Characters
Splitting and Merging
Find & Replace
Saved Searches
Clips
Clipboard History
Links and IDs
Styles
Indexes
Reports
External Editors
Python modules
Tutorials
Getting Started With EPUB
Prepare Your File For Sigil
Open Your File With Sigil
Save Your EPUB File
Add the Author and Title
Add a Cover Image
Create Files For Each Chapter
Identify Your Chapters
Generate A Table of Contents
Create Links and Notes
A Quick Introduction To Code View
Check For Spelling Mistakes
Check For Errors In Your EPUB
Edit With Find & Replace
Use Stylesheets
Include Custom Fonts
Advanced Find & Replace – Regex
Regular Expression Reference
Sigil plugin framework
Tips
FAQ
Common Questions
Where to Get Help
Using Sigil
Formatting and Styles
Contributing to Sigil
Python plugins
PS Hitch, I don't want to look as the teacher's pet, but you write very well indeed and your prose is often very funny, even your rants
.