There's plenty they can do to make the iMac a more modern, attractive device:
1.)
Bezels - Thin bezel displays provide even more focus on your content, and the bezels on the current iMac are oddly thick. In fact, while the 2012 redesign changed the thinness of the back side, the front face of the iMac remains unchanged from the 2009 introduction of the 27" and 21.5" display sizes. It's 2016 and 7 years is a long time in tech.
2.)
"The chin" - The chin of the iMac had been shrunken down with each iteration up until 2009, but is likely a lot taller than it needs to be given the advancement of other technologies. I personally would love to see an iMac with no chin - basically make it look like the Thunderbolt Display but with a computer in it.
3.)
Colors - Apple would have to be really motivated to add color options to the iMac line since they'd also have to add additional SKUs to the Magic accessories to match. I think the latter is probably why they won't bother, but the early days of the iMac was
all about color options. I hope they offer space gray and gold options with a redesign. I'm typing this on a gold MacBook and it is a very attractive look.
4.)
USB-Type C/Thunderbolt 3 - This is the future of I/O and Thunderbolt 3 is desirable for professionals who are a large part of the target market - especially for the 27" model. Apple has been the primary driver of Thunderbolt technology since its inception in 2011, so it's only natural they continue to push the standard on their flagship Mac.
In short, when Apple releases a redesigned 27" 5K iMac that hits some (or all) of these bullet points, my wallet is ready.