I've joked about it, but I don't foresee a 23" coming any time soon. Given the pricing of Apple 23" screens and the current iMacs, you're looking at a $2300+ (I'd say more) computer, and that's a helluva lot of scratch for a computer where none of the parts is reusable down the line.
The advantages of 23" vs. 20" are fairly small (esp. if the 20" is capable of expanding with another 20" - I've never tried it with the current model, does an ACD screen line up even with a 20" iMac?) for the cost. Nor do I think they'll have an iMac Pro with another hard-drive bay or top of the line graphics or more than two memory slots.
The highest end iMac is more than enough to suit the needs of most people who buy iMacs (esp. if you throw in a good GPU for people who wish to game), and though I'd buy an iMac Pro I'm not sure the market is there. When you start getting up in price people want reusable pieces (moving HDs from one tower to the next, reusing monitors, etc.).
