Hi Chris, sorry for taking so long to respspond. I'm not sure which part of my suggestion is confusing, other than perhaps the use of filter and smart mailboxes together.
A “smart mailbox“ (aka “smart folder“), is a definable repository which pulls together items of similar nature. “All unread mail”, e.g., or “No next actions”.
A “filter” is a way of limiting the display of a large body of content to a definable subset of that content, e.g. “From Jason” or “Available actions”.
The two are indistinguishable except in presentation. A smart mailbox/folder groups the results into a special location for reviewing at a time of your choosing; filters tend to act on the view you're currently seeing, showing and removing items as necessary.
The two can work together; you can create a smart mailbox of, say “All Overdue items”, and then further filter them to show only items with a time estimate under 15 minutes.
You note that the first two items, OverDue Items and Coming Soon, already exist. True, they exist, in that you can see items which are overdue, or which are coming soon. What you can't do is group them together in some meaningful way. Grouping and sorting by start date dont show me my overdue actions, nor do they show actions which are deemed “coming soon”. I currently have five items marked as “overdue”, and to find them, i have to scroll through my 106 active projects. A smart folder would put those five overdue items into a single folder I can easily click on and review.
If you've never used smart mailboxes (in Mail, Journler, iTunes, NetNewsWire and a host of other applications), I suggest giving them a try. They're a fantastic way to view slices of your data.