Ok, we know its 528 pages...
There is 270 pages in 7th edition.
Thats an extra 258 pages.
Lets say they dumbed down the rules like they have been talking about. Maybe 10% less pages. Lets say they removed all the summaries of stats - another 4 pages saved. Remove overlapping rules from machines/etc - Another 10% saved
So lets say there is 300 unknown pages.
If all 15 armies were to get an RH like entry, that would equate to around 20 pages of rules for each army. Which is a comfortable number to be honest.
Could it be possible?
What else could they add to the book? A GW pricelist? 300 pages of scenarios?
528 pages is pretty crazy. The current Warhammer Fantasy rulebook is 272 pages, and the Warhammer 40k rulebook is 320 pages. This number has been reported by a few different sources so I would say that it is pretty likely that the book will be this size.
There is a lot of speculation about what the extra pages will be for. The current most likely idea is that there will be a Ravening Hordes like list included in the book.