For a shared context, the basic elements of music are rhythm, melody, harmony, form, and expression. Studio One is very capable when it comes to most of those elements, including being able to arrange song parts and control instrument dynamics right in the sequencer. However, on top of basic melody and harmony exists a realm of functional harmony that is barely glimpsed in Studio One -- namely, when selecting a chord for the Chord Track via the circle of keys.
A Key Signature track could add a new level for music composition when used in conjunction with the chord track. A key signature puts notes and chords into a functional relationship with one another where each chord has a special role related to the root.
By way of example, there is a really useful tool called Hookpad that helps musicians write chords in functional harmony. Most notable in Hookpad is the ability to define chord progressions in terms of their functional role (via roman numeral analysis), colorize notes and chords by scale degree, and compose melodies through the selective use of stable and tension notes:
Similarly, the Captain Plugins suite allows musicians to define chord progressions based on the key signature and chord function and then create/randomize melodies based on the chord progression and the desired amount of tension/stability.
While Hookpad, Captain Plugins, and similar tools are really useful, it would be a much more streamlined and integrated workflow if Studio One were to offer similar tools for songwriters and composers. I honestly bought Studio One because of the Chord Track feature and would be very grateful to see a continued effort to enhance the process of writing functional harmony.