At this point, I'd be surprised if a 27" iMac is released again.
I can see Apple maybe increasing the display size on the existing model to 25" and calling it a day. This would be a pretty compelling fuss-free, all-in-one option for those that want it.
I stand corrected if rumours of a larger 30" iMac are true. Happy for those that have been waiting (so long!) for one. As long as it presents a compelling set of features and upgrades over the previous generation, and is reasonably priced, it should do well.