I am currently writing a forum CMS (http://pygm.us/IbkgNZ4d), and I, too, have wondered how to expand the number of tags within a rigid nomenclature. Using a lexer that automatically converts links to tweets and YouTube videos to embedded scripts seems like an unintuitive solution, but on the other hand, people unfamiliar with the commands will automatically display the embeds in their posts.
For commands that don’t convert links, I was thinking about something like ':<COMMAND>:' with an expandable list of whatever people prefer. Something Awful already use this model for their emoticons, and it could be extended to things like images in general like states for election conversation and such.
I have always found the `[url]` solution to be incredibly bothersome and to some extent unintuitive, and with the advent of mobile devices, letting people write their posts in as few characters as possible is a big advantage to be taken into consideration when weighing the pros and cons.