[Nukaru]

That's a little complicated.
A-side songs not on an 'album' are: No ni Saku Hana no Youni, Returner ~Yami no Shūen~, Justified, Jesus, GHOST, Journey through the decade, Koakuma Heaven, Farwaway, LOST ANGELS, Flower and the next decade.
But Justified, Jesus and Journey through the decade are on compilation albums or Soundtracks and the No ni Saku Hana no Youni & Returner ~Yami no Shūen~ singles were release 2007, so I don't think GACKT would actually add them to the next album (dont fit the theme of the newer songs, other style)

So that leaves GHOST, Koakuma Heaven, Faraway, LOST ANGELS, Flower and The next Decade for now.

He kind of did that already ^____^
The sixth day & seventh night, is kinda like a Best Album, with his best selling Singles on it. And I don't think GACKT would do that again, although he's GACKT, so you'll never know, what he tries for in the end xD


Edit:
Justified is a special track on the nine*nine live compilation, Jesus is on Slo-Pachinko Gladiator Evolution Original Soundtrack and Journey through the decade is on the Kamen Rider Decade Original Soundtrack.