[Baltan II]

A Millenium boxed set is absolutely possible, close to all of Heisei is too, but Showa is currently split up between too many companies to be feasible.

Sony currently has rights to all the Millenium movies (which is why their DVDs have been available and cheap for so long) and everything Heisei but 1985 and Biollante. They let their Showa rights lapse, though, which is how you end up with the insane prices on Son of G, Sea Monster, Hedorah, Gigan, and Mechagodzilla. It would also be a perfect time to upgrade the Heisei DVDs from the terrible double-feature discs available for most of them - grabbing the rights to '85 and throwing them all together could work nicely for Sony if that's what they wanted.