In addition to #6, which might not be fully scoped yet, I'd propose a very simple and effective solution - support editing the currently open file in an external editor.
For Android, at least, this would be as simple as supporting a few pre-defined editors (not that there are too many), and firing an intent to open them.
If something is missing on the receiving side, like accepting intents or reading the intent parameters, it could be contributed to another Open Source project (i.e. Orgzly).

Potential editors:

• GitJournal
• Markor
• Orgzly