I think, since the latest iMac was only released in May, I don't think there will be another iMac until Ivy Bridge is released. So don't worry about it.
I would be very surprised if there was another SandyBridge iMac released this year/early next year.
In terms of your second question; likely inclusions could be:Ivy Bridge
Integrated graphics chips + Upgraded discrete graphic cards
Perhaps more Thunderbolt ports