I am impartial to the future release being either a new album or a boxset. But here's my theory on why it might be a boxset.
Basically, the 4 main albums are chapters in BOC's life and each tells us what the brother's were thinking over the years.
After their happy days of experimenting with acid back in the day (their earliest EP is called Acid Memories for a good reason) BOC quickly realized the "irreversible vector" we were traveling on as a species and it quickly showed in the music. First there was Music, which explored the scary reality that childhood isn't always as sweet as we think it is but there are still genuine moments of bliss and happiness (ROYGBIV, Aquarius, Open The Light) Then with Geogaddi they faced the dark issues going on in the world for the first time, but it still had a warm, nostalgic, acid-y feel in places. Afterwards, there was Headphase where rather than face the ever darkening world, they created a place of refuge for their fans even if the ending to TCH was still inevitably sad. Lastly, with Harvest, they are finally coming right out and telling us that society is quickly coming unglued and they are weeping for it.
It took me a long time to realize this, but BOC's albums and their trajectory are perfect. They are an honest glimpse of society from people who have always preferred being on the outside of it. It is why I could understand Tomorrow's Harvest being their final statement and then going on to making archival works to support their kids. What more do they have to tell us about society and their feelings in album form? Now, we may finally get a glimpse behind the curtain and see the underside of their career, aka what they were working on between and during albums that never made it to the classic 4.
Warning: This numerology post is not to be taken 2 seriously.