Since Dave Winer is working on OPML 2.0 spec again, I thought I should write about an idea I had that'll help people discover, document, and reuse extension attributues. The idea is to create a wiki for OPML attributes and use wiki page URLs as namespace URIs. The wiki should have three URL types: namespace, attribute, and profile.
An OPML namespace wiki page lists attributes in the namespace. An OPML attribute wiki page documents the attribute and its value. An OPML profile wiki page is intended to capture common practice by listing attributes used in practice. A namespace qualified extension element in OPML head could simply point to the profile page attributes used can be understood in context of the application domain.
That's it. This suggestion requires no change to the spec and one optional extension element in return for a place where all OPML attributes and common profiles can be documented. I think it's a bargain. A step beyond this is introducing wikified-microformating to generate schema info from then pipe that to OPML validators so that OPML documents can be validated throughly without skimping on extensibility.