I frankly don't see iPad 5 becoming half as light and thin as iPad 4. That would imply 0.18" thickness (vs. 0.28" of iPad mini and 0.24" of ultra thin iPod touch) and 0.72. lbs (vs 0.68 lbs. on mini). Heck, it would be amazing if iPad 5 becomes 0.3" thin (20% thinner, same as iPhone 5) and weighs 1 lbs (30% lighter).
Having said all that, I would not be able to justify spending $170 more for 10-inch iPad when iPad mini gets both retina screen and similar processing power as 10-inch iPad. For this reason alone, I suspect Apple may add some new exclusive feature to 10-inch iPad.