I've been a long time reader. I enjoy the discussions here very much but I've never really had anything interesting to add. Here it goes:
I've noticed that if you use The Force theme as a ground then Rey's theme, Kylo Ren's theme and Darth Vader's theme appear to be contrapuntal derivations of each other. This means that they are "related" to each other, with "the force theme" as a parent musical idea.
The strongest evidence of this is on the piece "Jedi Steps", from TFA. If you recall, as Rey arrives to meet Luke there is a constant chord playing (G minor) with very melancholic theme on the bass. The musical recurse used is very elegant: the music starts in G minor and quickly changes into E flat major.
This only requires one note change in the constant chord: while the 2 bass note remains a G and a B flat, the higher note changes from D to E flat (just a semitone higher). The reason the chord tricks us into thinking that we are still in the minor and "darker" tone of G minor is because the notes in the lower register in the orchestra is still G.
Guitarrists know this kind of chord when they see a notation like "G/Eb" meaning "G in the bass, E flat chord on the top", also spoken as; E flat chord with G in the bass.
Suddenly, as Rey approaches Luke, the theme quickly changes into a quote from Darth Vader's march, which is ALSO an elegant change in chord: The harmony is G minor that quickly changes to E flat minor by moving two notes in opposite directions:
The G minor Chord in standard form (a triad) is G, B flat, D. To change it as described, the B flat stays the same while the top note goes up one semitone and the bottom note goes down only one semitone.
G--------> down to F sharp B Flat----->SAME B flat D--------> Up to E flat
This musical devices (voice leading, contrapuntal derivation, etc...) lead me to believe that John Williams is telling the audience that all musical cues referring to Luke, Rey, Vader, and Kylo Ren are related to each other. Maybe because they all share a connection to "The Force" or because they are all connected by something else... like blood.
So, in a nutshell: my theory is that they are all Skywalkers, and that this is evidenced by the closely related musical themes used in the saga.
Before you think I am completely crazy: this technic is around since Richard Wagner. John Williams is a composer very well connected to the classical forms, and demonstrates constantly incredible ingenuity and knowledge of music.
See this video below, with a simplified version of "Jedi Steps" played on the piano.
The same devices I described, along with others, can be used to explain how Kylo Ren's theme is also connected to "The Force Theme" while the themes used for the Emperor and Snoke are not.
Those are my two cents. Thanks for the good times and discussions on this board.
Edit:
I forgot to mention that the reason Rey's theme does not initially sounds related to the original "force" theme is because for most of TFA we are shown the theme transposed to A minor, fooling our ears into not connecting the themes together.
The trailer for TLJ does incorporate all themes in the same tonality.
