Devonthink already mimics index cards in fine style. Just write Markdown, RTF, or anything that will reveal what you wrote through quick look preview. There you go, a well organized deck of index cards.
There’s a workflow glitch, though. It’s not bad, and it could be cured with those most cherished words any developer ever hears. “All you have to do is…” 
Imagine you’re researching some topic. You write a note about some aspect of your findings.
Later, you learn something related that you would like to add as a subordinate note. Of course, documents don’t have child documents.
Your options are to write your new note and group the new note and the old note, tag the new note and the old note with a common tag, or link the old note to the new note.
But what if notes could have child notes? It wouldn’t take much.
Right now, selecting a group causes the preview window to display, “No selection.”
What if a selected group displayed “No selection” unless it had an annotation?
If a group had an annotation, it could be available in the preview area for display and edit just like a regular document.
Right clicking a Markdown or RTF document could include “convert to annotated group” which would create a group of the same name as the document, building the new group’s annotation from the original document’s contents plus its annotation, if it had one.
This can already be done, of course, but annotations can be overlooked. Markdown rendering isn’t done in the Annotations inspector, for example, although RTF is nice.
Surfing through ideas, if annotated groups are used throughout, would be more seamless. Click on an idea, if it has subordinates you can drill down with a consistent presentation of notes through items and sub-items.
Now you can have documents with children. Best of all, “all you had to do” was support preview of annotations when selecting a group.
If this is a viable idea, please let me know. I’ll go lounge on the beach, play a few rounds of golf, and in general faithfully execute all the duties of an end user throughout the development cycle. No worries, I got your back on this one.