I think Mithology will be buyed only by fans, once it's a 4-CD-set. Nobody who only likes few Bee Gees songs will buy it. Hence, I think it must NOT have all the hits, but it MAY include the forgotten highlights, e.g. "I've Come Back" in Maurice's disk, "Face to Face" in Barry's, "Gone with the Wind" and "Secret Agent" in Robin's, "All I Have to Do is Dream" in Andy's.
It may worth the price the fans will pay. Why do not release, instead a 4-CD-hit-set, a rarity set? It can even sell among not fans...
Rarities are by and large exclusively aimed at the fans, hard sell for the general consumer (ie the non-fans). Portioning those tracks out across the re-issued studio albums is probably the best way to go as it gives them more space as a single 4-CD box set would exclude far too many songs and also to keep those rarities in the context of the time they were recorded. It also ensures the continued work of remastering the studio albums since the Polydor/Universal releases are all out of print at this point and can demand a pretty penny on eBay.
They could on the other hand issue a series of rarities box sets to cover different eras of their career. I fear that solution would leave WMG free to not carry on with the remastering and probably ditch any physical releaseas of those altogether.
As in regards to "All I have to do is dream", given the history I doubt that song will ever surface on a Gibb CD. That said there was a various artists compilation CD issued in Brazil in the 90s called "You & I" which featured the recording but it is rarely seen, I do own one so I can verify it's existence.