YES!

1.) One way I can think of is upon right-click in the left column (this is possible already already for adding bookmarks, so it kind of fits). The user would then be presented with the options "insert opening fold marker" and "insert closing fold markers".

2.) By selection is another idea which I like. Select, then right-click selection, then choose "insert fold markers". (or select, then click in the left column)

3.) Paragraph-based ... not so much. I think people writing longer texts primarily need the markers to hide large chunks of text, not only single paragraphs.

4.) The text mate way of defining a special syntax to insert foldmarkers (as in the latex bundle: % (fold) and % (end) ) would also (kind of) fit with Ulysses. But maybe writers would not want to have "nerdy code stuff" in their texts.

5.) Yet another idea: Enable fold-markers with the paragraph styles. That is, each paragraph style would have a check-box "enable fold markers" . So one could have a paragraph style "headline" with "fold markers" enabled. The fold markers would then contain everything from the headline to the place right before the next headline.

Problem with 1. and 4.: This easily results in a mess.

My favourites are 2. and 5.

Nice!