Ability for users to edit an "about me" section within the user profile- using same structure as adding a comment.
system auto locking threads after X days, possibly moving these threads to an "archive" category of some sorts
Clicking the time of last comment on the index will take you to the last comment.
Icons for unread comments in each topic
Post Prefixes for categories. If a user starts a topic like: "[Prefix] Topic Title", the "[Prefix]" portion will be recognized as a link. When clicked it will return all posts with that tag. You could also translate these tags to css. See here: http://custom.simplemachines.org/mods/index.php?mod=4038
pagination- "load more" for pages 1, 2, 3, etc