I like the idea of at least two types of links:
"Responses", where a whole card is basically conceived as a direct response to another card.
The "referee" would include a Trackback-style link to the responding card at the bottom (similar to this pic). The responding card would include a link at the top similar to the way reposted cards are displayed.
"References", which allow you to just reference other cards. These could appear in the new card as footnotes.
Again, the "referee" would then include a link to the new card.