You should consider this list of items a good basis for the requirements for Mailman 3.0. It essentially contains a dream list of all the things that are too painful or destabilizing for the 2.x series.
Email Handling
Use VERP or DSN for address tracing, perhaps tied to the monthly password reminders, or VERPing the occasional regular message.
Re-implement bulk mailer the Right Way: an asynchat/asyncore server to do DNS lookups and remote MTA delivery directly (optional).
Plain text digests should conform to RFC 1153.
If a message has MIME parts and the header/footer is going to be added, the message should be transformed into a mulitpart/mixed with the header and footer added as text/plain parts.
Documentation
A detailed feature list
A user's guide
A site-admin's guide
A list-admin's guide
More on-line documentation and UI help
A developer's guide w/ architecture and API information
manpages for the scripts in bin and cron
Integrate Christopher Kolar's documentation
General Web UI
NO DEAD ENDS
All web UI must be configurable so that it more easily integrates into an existing site's design. Probably means using a template language/system like Quixote or PHP.
Default UI should add a navigation sidebar to all web pages.
A heirarchy of page designs and information: e.g. the most specialized of the following wins: site, virtual host, list, language, user.
Web pages should never mention turned-off features.
List Administration
Separate list admin role from list moderator role
Allow the moderator to edit posts being held for approval (make it evident, either through a header or other means that the message was edited by the moderator).
Allow "urgent" postings to all members by the list admin which bypasses normal digest delivery.
A button that will bundled and deliver a digest Right Now.
Allow the list-admin to require approvals for unsubs
Allow the admin to disable option settings by users
Ability to set defaults for the various user settings from the "Membership Management" page.
Allow admins to control and set individual headers, adding, removing, or overriding those in the original message (sometimes very useful, but could be dangerous!)
Member management page should display case-preserved email addresses (but still sort case-insensitively).
Member management page should work better for humongous lists, both in terms of performance, and in usability
Searching for members/addresses with regular expressions from both the command line and web interface.
New moderation choice: archive but don't send to list.
New moderation choice: annotate and send to author for resubmittal.
Make it easier for an admin who manages multiple lists to handling pending requests sitting on all those lists.
Ability to ban specific troublesome users (from posting, subscribing, etc). Posts from banned users would be discarded.
Better integration with moderated newsgroups (and allow some addresses to bypass even that moderation and be delivered to a secondary channel, like moderators@isc.org).
Ability to set the next digest volume and issue number from the web
Add an option to include Reply-To: header in the membership test for a members-only list posting
List Membership
Editing your user options should put you back to the edit-options page
Allow the user to be excluded from postings if they're getting them in the to: or cc: headers. Be smarter about filtering out duplicate deliveries.
Have one account per user per site, with multiple email addresses and fallbacks. Allow them to subscribe whichever address they want to whichever list, with different options per subscription.
Allow the user to get BOTH normal and digested delivery (but I still don't understand why someone would want this)
More flexible digests: index digests (subject and authors only, with URLs to retrieve the article)
Allow users to subscribe without selecting a password and have Mailman create a password for them.
Timed vacations, allowing a user to postpone or discard email for a certain number of days or weeks.
Site Administration
Allow the site admin to define list styles or themes, and list admins to choose one of the canned styles to apply to their list.
Full creation, deletion, renaming, etc. of lists through the web (and email?), including fixing aliases file updates.
add_members should have a switch to disable admin notifications
Other Usability Improvments
Allow individuals to turn off password reminders
Confirmations should be both email and web based, and be used for both subscription and unsubscription (configurable by the list admin).
A better strategy is needed for sub-lists and super-lists, including dealing with the resulting password reminders and authorization to modify the sub & superlists. Majordomo2 is reported to have some good features in this regard.
Add a limit on the number of posts from any one individual within a period of time (1 post per day, 10 per week, etc).
Don't use the first public mailing list as the `originator' of password reminders.
Mailcmd interface
Provide an email interface to all administrative commands
Allow email unsubs from matching address to unsubscribe, possibly adding an "allow open unsubscribes" option to control this. Also, adding a confirmation with hit-reply to resubscribe
Add -join and -remove addresses for easy subscription, unsubscription
For email subscribes, keep an audit of where requests are coming from, and send the original request headers in the confirmation message. Helps track down subscribe bombs.
Replace cron stuff with our own scheduling mechanism.
Get rid of the one-shot process model altogether in favor of a multithreaded monolithic architecture.
Better support for distributed processing
Use a real transactional database for all information, and allow various bits of information to come from different sources (a relational database, ZODB, LDAP, etc)
Keep a members Real Name with their email address
Member profiles
Do a serious and thorough security audit
Allow lists of the same name in two different virtual domains
More sophisticated attachment handling: strip and discard attachments, post attachments (e.g. via WebDAV) and rewrite to include URLs, etc. Should be admin configurable based on MIME type. Check out Bill Bumgarner's work on this.
Should include a --with-username=NAME option to configure (and not require it be literally "mailman").
Should be able to gather statistics, such as deliveries/day, performance, number of subscribers over time, etc.
Implement something like Roundup's nosy lists, maybe even integrate with Roundup.
Split Mailman into libraries so, e.g. the delivery part could be used by other projects.
Bounce handling
Make a distinction between disabled addresses due to bouncing and a user's explicit disabling of an address
Add more patterns for bounce handling (never ending)
Occasionally remove stale bounce entries
Send mail to people who are being removed without their knowledge (even though they're likely not to get it).
Reminders to disabled addresses. The idea is that if an addr is disabled due to bouncing, we should send out periodic reminders. We may want to do this for explicitly disabled addrs too, but perhaps with a different schedule.
Delete bounce disabled address after some period of retry
Pipermail + Archiving mechanism
Search engine for archives
Provide downloadable tar.gz's of the html archives
sort by date should go most-recent to oldest
allow list owner to edit archive messages
archive link should do *something* reasonable before the first message has been posted to the list.
optional form front-end to public interfaces as a filter to address harvesters.
clobber_date should support third option: only munge the date if the Date: field is unreasonable (far in the future or far in the past).
In general the whole Pipermail subsystem needs a good rewrite.
Code cleanup
Use all the new wizzy Python 2.0 features
Use the re module where regexp and regsub are used (not many left)
Refine any remaining unqualified exception guards to specify target exceptions, when appropriate.
Turn all remaining string exceptions into class exceptions
