As far as speculation, I always assumed (hopefully correctly) that one of the pricey whoppers would be a LETHAL WEAPON boxset. I know I'm not the only one who's hoping for it.
EMPIRE OF THE SUN is also a worthy candidate, and again, possibly just for my own sanity, I've always assumed it would be released under this banner as well, being Williams and a prestigious title.
The possibility I've been thinking more and more about lately is the film tracks of HEAT. The album, frankly, is brilliant and includes a very thoughtfully produced representation of the score, but the film tracks really let the material breathe (and Goldenthal wrote a TON of it). This could be a 2CD set even without the album being included.