Apple just released the iMac Pro, in June 2017 we saw new iMacs with better processors. Apple hasn't updated the look of the iMac significantly since 2012, but there are annual spec updates.
Expect to see a 2018 iMac with 8th Gen Intel processors that deliver more power to the Apple All-in-One. Apple last updated the iMac to Kaby lake in June 2017, so there is a good chance we will see an update in the summer or in the fall. The iMac Pro and the Mac Pro releases could factor into this as well as Intel's chip availability.
While the timing is potentially right for a new look, there isn't a dramatic need for a thinner iMac. It would be awesome to see Apple add FaceID and TrueDepth cameras for a Windows Hello like experience, but there aren't many rumors yet.