Version: 1.1 (KDE SC 4.5) (using Devel)
In the slicer configuration, you have a series of sliders to set various properties of the slicer in order to tweak the shape of the puzzle. But the way it is now provides no feedback regarding the end result for your puzzle, meaning getting things how you want it requires letting the whole puzzle being built then going through the dialog again, repeating the process until you get it right.
So my suggestion is to provide a live preview of the current settings in the slicer settings dialog. So it provides a picture of maybe four example pieces, and as you move the sliders the pieces change to show you the sorts of shapes you will be getting. As I understand it the slicer does not allow for creating single pieces, but you might be able to create a solid-colored 2x2 puzzle, or extract the inner 2x2 section from a 4x4 puzzle (so you have the plugs around the edges for examples as well).
An issue might be with the puzzle being randomly regenerated every time, with different plugs in different places. I am not sure it is possible to make the plug layout stay the same. It also may be too slow to re-slice in real time like that, I don't know the performance of the slicers (or if the performance can be guaranteed for third-party slicers). So perhaps the preview should only be updated if a slider hasn't moved for a certain period of time (perhaps with some sort of "waiting" message or something so someone knows the changes are not being displayed).
Reproducible: Didn't try
The preview should be also when selecting the slicer in the first place.
That occurred to me, but that doesn't really need to be a live preview, it can be a pre-rendered image.
Another idea: have an option to review the result of slicing before accepting, so that if you don't like it you can go back and tweak things without having to start over (and remember what options you used, etc.).
A "sample piece" preview is probably most useful for regular grid slicers. The above is probably better for e.g. irregular slicer though, since sample pieces are less representative.
Any update as I saw the same issue happening at my end. Because there is no live preview there isn't a way to make information-based decision.
It should have previews for all different slicers and how the puzzle pieces will look. It doesn't have to render the image that we are trying to make into puzzle but from one of the representative images which are already part of palapeli.
Created attachment 112685 [details]
palapeli slicer info. no image hence it's trial and error method.