How about a new Cardtype for this? Maybe 'Link', so when I do this: {{My Link Card}} the contents of My Link Card is a URI, and it gets formatted as a link to that URI, with default link text being the name of the card. I think that is a really easy module, and could be added to the standard module (likely a good idea). The editor for this type would probably be just the Phrase editor (by inheritance), and the link/inclusion editor could give the option to create a Link card when creating a link.
Philipp's exploratory PR: https://github.com/wagn/wagn/pull/165
Perhaps Philipp's idea + Gerry's solution + "everything is a card" is leading us to this: every URI is a card. Or at least, we make it very easy to track URI's as cards, such that if I have twenty links to wikirate.org on my site and wikirate.org rebrands to wikigreat.org, then we can update them all at once. This would also make it easy to do things like link tracking (eg, for your wagn to track how many times someone clicks from your wagn to wherever else)
To me this chunk of functionality fits very well with the link editor functionality that is coming up in 1.14. Gerry has already put together a pull request with the basics of this idea. I would lean towards doing it in 1.13.1 or something, because 1.13 has been a beast to get out the door and I'm already nervous about the size of our migrations, but generally I think this has the looks of a solid approach.
Just merged the URI card in this pull request. I really like the idea of automatically making cards from links. Some link forms don't lend themselves to automatically creating a cardname for them.