I've prepared a list of 'simple' improvements and additions which I think would be useful.
I may have not noticed existing facilities that can be used to achieve these (despite looking closely) so if anyone can give comments and advice that would be helpful. Also, if anyone can suggest changes or additions?

If nobody shouts "stop!", I'll put these in as a Wishlist bug in the near future.

1. The Image -> Resize Canvas tool gives fine and detailed adjustment and has a small thumbnail type indicator which can be dragged for coarse position adjustment. However, there is no feedback outline shown on the canvas which would be a useful improvement if it could be done.

2a. Further to 1, a Resize To Selection function would be useful for easy and detailed adjustment of the final canvas size and position. This would be similar to Trim To Selection except that it wouldn't remove any content.
2b. If this Resize to Selection function initially brought up the Resize Canvas tool, with values set to those defined by the selection outline, the Resize canvas tool could take over for any final numerical adjustment, with its feedback outline shown on canvas.

3. Another function could be Resize To Layer. This would be similar to Trim to Layer except that it wouldn't remove any content. This would enable you to set up individual layers (under the background layer probably or anywhere you like) that define 'viewports' in the form of rectangular filled areas (or non-filled rectangles) which could later be used with Resize To Layer to redefine the canvas size and position.

Wishlist report made: https://bugs.kde.org/show_bug.cgi?id=415600