I think I would prefer if books like this were done in the same manner as the PHB's in a sense. That is to say, have the first Manual of the Planes focus on the Shadowfell, Feywild and Far Realms each in ample detail, with stats for the major players in each, info on locations and of course monsters, paragons and epic destinies. Then have the next Manual of the Planes be more like the Plane of Dreams, The Nine Hells and The Abyss, and again go into all the details, etc etc.
Idk, I much rather them throwing us big, juicy portions than samples of everything. I mean its not like if they released a Manual of the Planes entirely on the Shadowfell, Feywild and Far Realms that they couldn't release yet more supplements regarding those realms, like something aberrations focused or fey focused, or expand it in the future Planescape or Spelljammer settings.