It would be handy if someone could essentially create a custom questions time line. Ideally it would allow questions from more then one tag to be displayed at once.
For example, if someone's interesting tags include PHP, jQuery, and ecommerce, they can create a time line that show all question tagged with PHP, jQuery, or ecommerce. Basically if a question has one of these tags, even if it doesn't have any of the other tags, it will be listed in that users questions time line.
This would make it easier for those of us who only want to follow a specific set of tags. Especially when that number of tags is greater then a handful. Even better when many of those tags get very few questions. It would beat having to open up each tag's timeline every time we log into SO to see if new questions have been asked with that tag.