What if "http:" was a namespace like "user:", you would use [[namespace:<data>]] for all sorts of links and that would be it. No more bare URLs and no [URL] with single brackets.
Hmm. That might work too, and would be even simpler. Thanks for the suggestion--I solicit comments on that as well. I don't think it would be too bad, though, to have an "external/internal" separation between one-bracket and two-bracket links if your idea has some technical problem I don't see right now. That's pretty simple to understand an use.