expando-tags: tagging (as a form of filing), aka keywords, would be easier with, say, abbreviations that immediately and visibly expand into a longer tag, or several longer tags. This can sort of be done already using the browser's built-in autocompletion, you just have to type in an abbr. and its expando-tags first, then anytime later you use the abbreviation they'd come up, and I guess you'd then delete the abbr.
Tag macros: rules that add or remove tags according to the existing tags and other facts about a post. I think I can do this in a little Python with the del.icio.us API.
No, I wouldn't recommend this for general use, only for the twisty people.