I wish there was a big worldbook for Rifts, but with no stats in it. Just lots of text and lots of cool art. Keep the era near the beginning of the setting, before all the Tolkien metaplot garbage. I'd buy this book in a second.
I used to really like the Palladium system, as it was obviously based upon AD&D, but with guns and robots and magic and psychics and tons and tons of awesome stuff. It really is my favorite published setting (except for maybe gray box Forgotten Realms). It's a shame the rules are so mediocre (they are not horrible, but they are not great).
A book without rules baggage would be great, and I suspect it would be a good seller for Palladium. Sadly, I don't think Siembieda would ever consider it, as he really does believe his system is the best.