This is for Redux but it probably applies to Renoise.
Let’s say that you are slicing/remixing an acapella track and added lots of markers for experimentation, you have then sequenced some MIDI, are more or less happy with the arrangement but want to add some extra markers right at the beginning or the middle of the sample.
With the current feature set you are up for a chore when doing so, as notes on the whole existing arrangement play now a different slice. They have to be transposed by a non-constant amount on the note range on as many separate MIDI items as you have.
Note that on this use case, which is slicing a full track, the button to to not stop playing when hitting the next slice is triggered, so destructive rendering is out of the question, as is e.g. you make 40 slices you might be increasing the patch size by 20 (40/2) on average.
So the real solution would be to allow free note mapping of slices as if they were individual samples, but if for some reason this isn’t possible, adding a stable-slice button that assigns the first free key/slot on the range to each newly added slice could be a workable solution too, as previous note mappings would be kept stable.