It would be great to be able to add instruments or samples anywhere in the song. For example, if a song has 10 samples and you want to add one after sample 4 or before sample 3 it would be great to have a command (perhaps iin the tree view) called Add sample or Add instrument (depending on where the click occurred). Clicking this menu item would bring up a dialog allowing the choice of how many instruments or samples to add (up to 4,000 - song amount for samples and 255 - song amount for instruments) and whether to add them before or after the selected sample or instrument. This would then need to renumber the samples or instruments after the newly added one(s).
Edit: Moved to the issue tracker (http://bugs.openmpt.org/view.php?id=88)