Creating a mixture of samples and instruments is tedious and oftentimes confusing with the current code. Additionally, it is possible, under specific circumstances, to define a "Pattern Editor instrument" (that can be called in the Pattern Editor but doesn't exist as an Instrument per se), which is merely a sample call. This would be indicated by an "s" prefix in the Instruments menu in the Pattern Editor when other "real" instruments exist.
What we need is a clearer way to set up sample-to-instrument declarations. Yes, drag-and-drop is pretty simple, but it would be good to have the sample and instrument slots all set up from one place.

Attached is a mockup of my idea.

You would be able to view up to 16 sample slots at a time, each 16 separated into tabs (which increase as you add sample slots). You can also sort the slot list by Sample ID or by instrument ID.
Each row holds the ID of sample slots, sequentially if sorted by Sample ID. The first field in the row is an editable box which allows you to name/rename the sample. If the sample slot is not present in the file, the sample name is blank and the rest of the controls are disabled. If you wish to assign the sample to an instrument slot, you check the button between them to link the sample to the instrument, then open the combobox under the Instrument Slot list in that row and choose the Instrument Slot ID, which always has one plus the number of assigned Instruments. (That is, if you've already created 2 instrument slots, you can choose from 1 - 3, where choosing slot 3 creates a new Instrument Slot.)

For multisampling an instrument, you can do a quick sample mapping in the Note Range comboboxes (which become enabled only when the link button is set and the Instrument Slot is chosen). Of course, more detailed sample mapping can be done in the Instruments page.

If you wish to assign a plugin to the Instrument Slot (whether or not it is sample based), you can choose here from the Plugin List combobox.

The only drawback is the use of null samples (sample slot 0) which you would normally assign for instruments that are plugin-based. When sorted by sample, the null sample cannot be shown, but normally these are created in the Instruments page anyway. In the above example, in order to designate a plugin to an instrument, 3 extra empty sample slots are created and marked "none." This is not a problem -- until you use the Clean Up function, which will eliminate the empty slots, but the plugin-based instruments would be intact, just not visible in this dialog -- until you sort this dialog by instrument. Thinking about it now, it might be useful to include an icon for creating sample slots when none are assigned. Care must be taken to keep this dialog simple, however, as we don't want to transplant the Sample page to this dialog. The whole object here is to quickly define the sample list and link samples to instruments or instruments to plugins.

This request is easier said that done, i'm sure, but we're not adding new functions except those that deal with GUI-to-data input.