just on-the-fly alteration of the frame aspect ratio, leaving the underlying image undistorted! I suppose it would be a bit like auto cropping while adjusting the frame shape, with subsequent readjustment of the image position within the altered frame being carried out much as it is now, in the layout page. In a way the functionality exists now, with the dimensions of the frame being able to be adjusted numerically in the layout page. It's just not as intuitive as tidying up a layout by dragging corners of a frame.
To change the print aspect ratio you do one of the following:-
1 - If you have various pre-set image sizes set up; eg. 100x125, 100x133, 100x150 mm etc. you can change the size (& therefore aspect ratio) on the page preview by selecting the image, shake mouse or long RMB to bring up Print Properties and select new size. Crop button on of course.
or
2 - Same as above but from right mouse click in Editor screen, select New Size.
or
3- In Editor screen, select image, use Size & position tab, change one dimension by direct entry and click Apply to Selected or All. Click the crop tab to adjust your crop.
You can drag he corners of the image in the Editor but that changes the both dimensions but because the aspect ratio is determined by the print size, allowing dragging of one side would be distortion so what you suggest does not seem logical!
No. 3 above is the most flexible and not difficult
EDIT
just on-the-fly alteration of the frame aspect ratio, leaving the underlying image undistorted!
I think I see what you are getting at, tricky!
Terry