OPML has served us well, but for OO4 I would like to see a File > Save > File Format > FoldingText

The FoldingText outline format (nesting fully-encoded in plain text with markdown and tab-indents, and columns/attributes encoded as @key(value) tags, is just so much more humanly useful and tractable than angle-bracketed XML, with all its opacities and fragilities.

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Not to mention, of course, OPML's inability to transmit bold or italic emphases, the difference between headers and bullet points, or the distinction between bullet sequences and numbered sequences.

It can also be a little unclear about the relationship between linked text and the underlying URL - try an 003 → OPML → 003 round trip with an outline containing links ...