A pop-up editor would be nice, but even a viewer, something similar to Okular or Quicklook for Windows (I know there is a lot better viewer for Mac, this was just two examples that I use myself).
I think there are multiple text editor libraries that support both RTF and Markdown, but programming for me is just a theoretical knowledge… i.e. I can read the logics, but when I try to do it myself, it seems that I never get it right… 
Personally I use markdown tables a lot for lists, so supporting preview of them would be nice, editing them is not necessary, we can do that in the Markdown editor of choice…
So an export/import or sync would be nice, you could add the important data you need to the YAML header of each file…
A simple viewer that will display the content of a linked file of correct format, and that can open the linked file in a default editor if needed, I think is more than enough for most users using markdown files…
And if you use the right viewer/editor, it would even display linked pdf’s, docx, etc.
Click a link or button to open it in default editor, do the changes, save it, click “refresh” and the preview in Aeon will show the updated content… (If you don’t find a way to auto refresh.).
Back to sync
To be able to sync both a folder structure and content both ways between a folder with markdown files and the content of the files would be amazing, and you could use YAML to define Properties, and the body of the markdown note could be text in a text field for the editor you mentioned.
If it was a push-pull auto sync, it would be possible to edit the notes in your preferred markdown editor while timeline is open and the changes would be updated IRL time, that way you could actually use the markdown editor as a companion to Aeon…
(Yes, I do understand this is a lot of technically obstacles to plow through, so it is just some thoughts for a “perfect” solution).
As an example of how I do it today and did it in version 2.
I create a link to a file, i.e. one of my markdown research notes for that object (item), If I don’t have any research notes for the object, I will create my folder structure of:
- Research Journal for {Name of object}
-
- Research Logs for {Name of object}
-
- Research Plans for {Name of object}
-
- Research Notes for {Name of object}
and add a Markdown file for the research objects where it belong…
I also distinguish between research notes and other notes about the object, and can also have my own notes in a research- journal/log/plan/notes, that is not a research note…
It is a lot of extra work, but I have “lost” so much information because I have written it down “somewhere”, never to be found again… and it is even more work to find that information again, if even possible…
Even more digression
All this is also why I applaud any software that have some good interchangeable formats or even implement interoperability to other tools… I really love the idea of “open data open source”, Open Data being the most important of those…
(I am so good at really spinning way of topic)