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.