Hi Guys, I LOVE Agenda very much and am ready to support you by purchasing the current premium features 
my gut feeling is that the app is going to get momentum as long as its goes in the right direction.
I recently installed Joplin (open-source + desktop + mobile) and gave it a go as a source of comparison. There are very very nice things in Joplin as well, and to me it is like an open-source cousin of Agenda. The nice features in there:
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you can create a note, but you can also create todo items. Translated to Agenda, it would mean supporting 2 types of “cards”. You would have cards for notes - as now - possibly linked to Calendar events, but also cards for todo items, that one could link to reminders, and also date ranges in Calendar.
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Joplin supports markdown at 100%, even LaTeX (which is an absolute must-have for scientists like myself). However, it contains an editor side and a preview side. But being 100% based on MD is such a big plus, that I think Agenda should be as well. In your case, though, the best option would be to avoid a preview side and have the MD content rendered within the cards themselves. A card would be pure MD in edit mode and rendered in view mode.
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Joplin supports various operators to sync files, in particular owncloud, so that SMEs like mine can operate the app on their own.
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Joplin provides end-2-end encryption (but not in a collaborative setting).
Overall, I feel that you guys might take a look at it and import these nice features in future releases. What do you think?