Retina. I highly doubt we'll see retina screens. I just don't see a need for them for a desktop computer. Everyone thinks they need the latest gimmick, but Apple is usually pretty good about controlling themselves in this regard.
Storage. I think an SSD standard probably isn't going to happen either. The desktop user is a different crowd than iOS and even laptop user. More storage is not only expected, but necessary. Though if they can get the price down on pure SSD drive significantly enough as to offer a 1TB SSD for the same price as the standard 1TB HDD, then it might happen.
Design. Mehhhh, I don't think so. I'll side with Derek on this one. If there is a redesign, it'll probably be slight.
New Sizes. If there is a new size, I think it'll be the replacement of the 21" with a 24" LCD. That being said, I doubt Apple would do that without upping the 27" to 30". Right now, there is a clear line drawn between the two sizes. Most people know they either don't need the 27" or do need it. If you move up to a 24" screen, you blur the line. I suspect they would lose quite a bit of sales of the 27" to the new 24".
What I would like to see, no matter what size screens they offer or whether they are Retina or not, is the removal of the giant black bezel - or as much as possible. A nearly edge-to-edge screen would be nice.
