Well, I think they will definitely keep the internal optical drive, just too annoying to cart around an external bay.
I think it could get a little slimmer, maybe similar to the 12" PowerBook's 3cm compared with the iBook's 3.42cm. Maybe a little lighter ~2.0 - 2.1kg (current 12" iBook weighs 2.2kg) although the machine will be a bee's bigger than the 12" so it could just stay the same weight.
I think we'll see a similar-to-Mac-mini option of a Core Solo in the cheapest model and a Core Duo in the more expensive. Or, they could up the price of both of them, use a 1.66GHz Core Duo in both and differentiate the two on other features.
The innovation I'm really hoping for is the screen, I can't see the screen outside on the dimmest of cloudy days here so I'm hoping for a real ramp up in the brightness/sharpness/colour of the screen. None of this glossy "Hey you, get out of the way I'm trying to see my computer screen! Oh wait, that's my reflection..." crap that seems to be on a lot of PC laptops nowadays - I want a really good display in this laptop. Not that I'm in the market (Damn you iBook, you died twice before, why won't you die now?!!..I'm kidding, really, no don't get mad...) but if they put in something that knocks the socks off my current screen it would be reason to celebrate as I would now have something for which to aim.
Other than that, I don't really care about graphics, so long as it can output HD onto a nice big LCD/Plasma display á la the Mac mini Core Duo and rip my TV DVDs to little files I can watch with Front Row I'll be happy.
Oh, and a bloody DVD burner - I really wish I had one in this, you should see all the CDs I have lying around I could fit a season of my TV shows onto one disc rather than 8 and that alone would be worth the hassle.