Ideally, it should be outliner meets editor meets to-do-list

Outliner:

  • Simple entity structure like WorkFlowy where everything is a node; not a notebook vs note vs to-do distinction; See Outliner (the app) or may be Orgzly
  • Support for infinite listing
  • Seamless sync across devices - untested
  • Powerful search features with tag support
  • Collapsing and expanding any node
  • Focus on any one node at a time - like WorkFlowy

Editor:

  • Support for attachments
  • Support for longform text
  • Markdown
  • Auto time-stamping

To-Do List:

  • Due dates
  • Reminders
  • Prioritization

Misc.:

  • Open source and free - as in freedom
  • Affordable
  • No forced-cloud-hosting; option to use 3rd party providers like GoogleDrive / DropBox or own cloud
  • Hand-writing support like in FiiNotes - best so far