I very agree with your approach to create a new piece having a musical idea in mind, if not yet a melody or motif to be used as the main subject, but a rather „technically“ one – here your choice of the interval pattern you described. Even if you did not invent a new chord or a new scale, this is a unique, this interval pattern is a „unique selling point“ of the piece and creates the mood of the piece which is indeed „ethereal“. And with your realization and recording so far, you have really caught this melancholic feeling with the warm timbre of the cello and the soft piano. I especially liked the small details such as the grace notes, the arpeggiated two-note-“chords“ and the triplets. Now, to get the piece continued and finished, I think it’s time to think about the form. Since it is already lengthy and although it has just separate sections, the listener is somewhat lost not exactly recognizing the structure and find out where the climax is. And in that sense it becomes a bit repetitive because there is a lack of contrast to the overall calm and „airy“ mood. Therefore, I would suggest to consider to put the piece, for example, in Rondo form where you could use the existing material for the different A (or A’) sections and there were room to introduce sections with a contrasting mood (in the B and C sections). For such a contrast I could imagine passages with a more dramatic expression or a final, triumphant resolution. Another possibility would be to have a section with a more distinctive and memorable melody (e.g. a „real theme“).