One attribute I desperately need is an external_id attribute in MediaWiki's user table. I'll probably try again to get this added to the schema.
Out of curiosity, what was the objection last time?
I didn't officially get denied. I asked the developers in IRC, and got a few "it might be ok", and a few "your plugin should just create a table".
I'll ask around again, and if I get any good responses I'll put in a ticket. I'll have to find a developer willing to commit a schema change though; I don't like to muck around the core code, and very much don't want to make a schema change.