Hm, Idk, I have a hard time imagining what they gonna put on it apart from some songs which will certainly be there.
Someone on another site mentioned this is supposed to be an All Time Best, but that info is not on her official site, so I don't know if this is true.
But at least then it would be more obvious what will be on it, like CD 1 songs from M Best I and the 2nd disc newer releases.
Btw. she just started her 15th anniversary tour this weekend. Here's a report with pics:
https://mdpr.jp/music/detail/1869301
It only mentions that M Best II will be a summary of her artist activities of these 15 years.
Could be an All Times Best.