Panning Slide effect:
Being able to put this in a pattern would be useful, even if you are using panning envelopes for instruments. Normally, you would require duplicates of the same instrument with different envelopes if you wanted to give it a different panning effect, but by implementing this you could drastically reduce the number of instruments that a song could potentially require.

Channel Panning Slide:
Same as above, but effecting the channel pan.

"Set Relative Panning" option for samples and instruments:
Being able to choose whether you want to set the "absolute" panning for an instrument (the current system, which overrides channel panning and most panning commands), or the "relative" panning (for instance, +2 or -2)

"Surround" checkbox on a sample-by-sample basis:
I am certain that this will come in handy, especially when you are working with only a small handful of channels that are filled with notes.

Allow panning when surround is enabled:
Sometimes you still want something to move left and right, even if the right-channel is inverted and the left channel isn't. To prevent cancellation, all you have to do is just make the left channel quieter when panning right and make the right channel quieter when panning left.

Have the lines in the sample editor change color to indicate clipping (like in Audacity):
A simple change. A visual indicator that a sample is too loud (before you actually playing it) would be a very handy thing to have.

When saving a module that was previously an MO3, have an option to leave the samples compressed (assuming the waveform hasn't been modified)

When importing an MP3 sample, have an option to leave it as an MP3:
You don't magically get an increase in quality by converting it back to a FLAC or WAV, so why waste the extra space?