I have been in the market for an outliner that outputs to open formats so I am absolutely your audience. (My current top contender is OutlineEdit, and I was days away taking the plunge so thank you!)
Two things I would love to see in any outliner:
1) Support Harvard and other outline format. I grew up outlining in Harvard and this bullet list stuff is maddening. The developer told me that OutlineEdit will not support it, period, so it's not a deal-breaker.
2) Horizontal outlining. Sometimes it's nice to have a hierarchy that expands and folds to the side. There was a Mac app called Tree 2 that did this but it is long unsupported.
Daily TaskPaper guy, so this is a great development.
I have thoughts (no plans yet) about plugin API that can use web view as output layer. The idea being that an outliner can be used for many things. For example you can create a calendar where the hierarchy is year/month/day. Neat, but not really a perfect solution for a calendar. So the idea with plugin API + Web View is that you could use that outliner calendar data to generate a nicer view. This keeps the outliner simple and uniform, but also gives you data specific view.
I think that same solution might work for Tree style view, or Harvard bullet list view. It would be sorta like a markdown app, content on one side (the outliner), generated polished result on the other.
No promises, but that's how I think I would implement those features in Bike.
Two things I would love to see in any outliner:
1) Support Harvard and other outline format. I grew up outlining in Harvard and this bullet list stuff is maddening. The developer told me that OutlineEdit will not support it, period, so it's not a deal-breaker.
2) Horizontal outlining. Sometimes it's nice to have a hierarchy that expands and folds to the side. There was a Mac app called Tree 2 that did this but it is long unsupported.
Daily TaskPaper guy, so this is a great development.