I doubt this will have any UA material in it. UA is all variants, whereas, I bet this book will be add-on rules that don't require any low-level changes to existing/core rules.
I also don't think this book will have anything from Tome of Magic, Magic of Incarnum, or Tome of Battle. Those books are self-contained cap systems that need a book of their own.
There are also too many feats to do them and something else in a book. I don't see any reprints in this book. At least, I hope there aren't any feats.
Instead, this is probably one of two things. Option one: new mechanics spread all over kingdom come; like magic sites, environmental rules, organizations, teamwork, spell book materials, etc. Option two: a collection of base classes, prestige classes, alternate skills and skill uses, races, and equipment found in the various complete, race, and environment lines, as well as some FR-specific stuff like in the Spell Compendium (and, presumably, the Magic Item Compendium). It could also be a combination of the two, but I suspect that'd look like option two, with some bonus features.
Personally, I'm looking forward to this, especially if it's option two. There is enough extraneous BS is the various books that I just don't want to deal with anymore. It's not that I don't like the stuff, I just want my library organized by function, not theme. I'm starting to look at the compendiums as chapters is a rather large PHB/DMG3. The "Rules Compendium" will, hopefully, fill in all the smaller chapters for me: races, classes, skills, description, equipment, combat (okay, not so small), and adventuring.
Once the Feat Compendium is released, I'll have my whole book and can cull the complete/race/environment books completely. Then, I'll just be waiting for 4E so I know my collection is complete. Actually, I'd continue to buy cap systems, like ToM and ToB.