Quad core would be interesting in an iMac, I don't think it's impossible. It's quite possible for Apple to do it, they often take high end stuff and put it in consumer level devices, I wouldn't be surprised. That won't mean that the iMac would become a Mac Pro: The Mac Pro might as well get 6 core processors, as we heard in a previous rumour.
As for Blu-ray, I really don't care for it, I always think that in the future we will abolish mechanical storage in favour of solid-state storage as it gets cheaper. Disks are loud, take up lots of space and battery power (laptops). But they're cheap...
What would be interesting in the iMac though is a newer design: I think that the bottom thick part could disappear, making the iMac basically a cinema display with a computer inside, but without making it much bigger. I don't know whether that's possible, but I'm pretty sure they'll do it as soon as they can.
And I'm sure Apple wants to put multi touch into everything, so the iMac has go get multi touch somehow, probably through the new Mighty Mouse and/or the keyboard.