A simple solution might be to add a custom filter box to the email tab.
Currently we have:
Filter: [By State]
[View All ]
[Read ]
[Unread ]
How about:
Filter: [By State]
[View All ]
[Read ]
[Unread ]
[my custom title 1]
[my custom title 2]
[+ Custom Filter]
If + Custom Filter is selected then a popup opens that allows you to give the filter a title and set a few parameters like: to, from, subject string, body string, tags, maybe some relative dates like 'last 7 days', 'last 30 days' etc., and a checkbox to "Make filter available to other users" which would by default be unchecked.
Submitting the form in the popup adds the filter to your drop down list. Of course there would need to be an additional link (and corresponding page) to allow you to edit and manage your filters.