For remixing songs I would to see a feature which allows the saving of individual pattern editor tracks or groups of tracks as a file which can be saved from one song and loaded into another. It would be good if the effects chain for the track was also saved along with any instruments and automation programs which appear in the track. This feature should allow you to export a specific section of a track from within a song, say the track from the whole song, current block, selection etc. The same system could be applied to a selection of tracks which can be exported to a file and loaded into other songs as necessary.
The system would benefit from a preview or pre-listen facility so the user can check that they have selected the correct track file for adding into their song. This would be particularly useful for things like creating and storing break beats which can be reused.