I really hope that there won't be tracks that already surfaced in the studio albums. I really see no point of that. I bought Crossroad because of the 3 new tracks and even then I felt that greatest hits album are like a bit ripoff for the fans. Its goal is for the new people.
However, boxsets are usually intended for the diehards. I'd rather have the b-sides, demos, acoustic versions and rare live stuff. I'm sure they got enough to fill in a 5 disc set.