Having a similar facade is not the same as exactly the same design... Going all the way back to 2004 counts even the white plastic iMacs as the same design, which pretty much means "any all-in-one with the hardware behind the screen, regardless of design or construction, is the same design". By that standard, no, we'll never see another iMac redesign - it's pretty firmly stuck as an all-in-one, and Apple has experimented with hardware in the base and decided to go behind the screen...
What would you like to see for the front facade? If no bezels/chin, that means laptop chips and/or very low-powered GPU as well (assuming it's not thicker, which is an Ive barrier - wish all you like for a thicker iMac without a chin, but Apple will need a new design chief before it might happen)...
If the bezels and chin have to stay, how much can the rest of it change? It's otherwise all screen, and monitors tend to look alike when off. A new display could mean even higher resolution (possibly including new sizes). which has already happened three times in the lifespan of iMacs with hardware behind the screen - 17"/20" to 20"/24" to 21.5"/27", plus the upgrade to Retina. It could mean Adobe RGB (although Apple seems to prefer DCI-P3), a brighter display, or, at some point in the future, OLED. Barring something like a 31.5" iMac with an 8K OLED display (nice, but do you want the $8K price tag), most of these upgrades would be dismissed as not being a redesign. If that 8K iMac came, would you want it without the bezels and the chin? What if it meant we got the processor and GPU from the top MacBook Pro (a 6 core mobile i9 with a midline Radeon Mobility GPU) instead of a fire-breathing Xeon with up to 22 cores and a desktop Vega? The iMac has enough cooling problems without taking away the spaces where Apple hides a lot of the cooling...