I've just sent in two gimp ui brainstorm pictures for angled guides to
that gimp brainstorm blog, where casual comments are discouraged, so I
thought I'd summarize here:
1. Angled Linear Guides
Double click on a line defines a center of rotation. Drag anywhere
else on the line, and the guide rotates around that point. Pressing
shift, control, or shift+control can change the precision of the
rotation. Double click also undefines the center, or redefines it.
When there is no center defined, plain dragging will move the line.
Dragging with shift, control, or shift+control, will still rotate the
line, but rotates it around where the button was clicked down at. The
amount of rotation in this case could be, for instance, based on moving
horizontally back and forth, or perhaps have the rotation center be
arbitrarily assigned to be a little ways away from the click point (but
still on screen).
2. Guides based on any path
This would make precisely aligned touch-ups easy, especially with
tablet pressure effects when painting, when "Stroke path" just doesn't
cut it. Double clicking could enter and exit a path editing mode.
Think Inkscape engraving tool for potential expansions of this sort of
Gimp-developer mailing list