Well going the way it is (Look at the 2016 MacBook Pro).
New iMacs will definitely:
1. Have USB-C ports (with Thunderbolt 3 support). So you will need a lot of dongles.
2. They MAY remove Ethernet port (maybe not?)
3. Remove expandable RAM?
4. Will definitely remove SD card slot.
5. Keep Skylake processor (as it is now, with i7-6700k) or get early Kaby Lake processors, which ISN'T a huge jump in performance, maybe 10-13%).
6. Brighter display (just like the 2016 MBP)
7. Faster SSD (it's already pretty darn fast as it is) but the price will go up on these SSDs.
8. New Polaris based GPU (Or Vega??). Newer AMD GPUs are more efficient in terms of heat, but they are NOT performance monsters like newer-gen nVidia GPUs. I assume it will be a 4GB or 8GB GPU configuration with an RX480 or similar Polaris GPU.
9. Thinner
10. Touch ID or TouchBar (probably on the wireless keyboard?). Touch ID might be on the rear, I don't know if having it on the keyboard is "secure" enough for them.
So yeah, I don't think it's a SIGNIFICANT update for editors or graphic designers. The GPU is probably the only thing that's a worthwhile update, but even then, Polaris is NOT a revolutionary architecture. Vega is supposed to come in 2017, so maybe Apple already has dibs on it.
Also keep in mind the next update will be a redesign, so we have to deal with first-gen issues, which are typical with Apple (anyone remember the first Retina MacBook Pro? Fans used to kick up very early and get loud, and heavy throttling would lag the whole system)...
or 2014 iMac which has thermal throttling, that the 2015 doesn't.
So anyway, I have the 2015 top of the line iMac in my posession right now, and intalling Adobe, Apple, Davinci Resolve apps at the moment, will report back on the performance.
I know for a fact that FCPX is heavily optimized, and it will run like a beast.
Premiere Pro on the other hand (even CC2017) is a bit of a hit and miss. I think they work best with CUDA based GPUs like the Quadros with lots of GPU VRAM. They fly on PCs (ew) with higher end GPUs. You can slap on multiple LUTs, transitions, filters, layers, tracks at 4k+ and not have it choke. But on Macs it's a bit of a mixed back. However, Adobe started using Metal (instead of OpenCL) now so that may change things in the future.
Will report back. I'm setting this up for a relative who's an editor, so I have to run a few tests. I told him that he can wait until March for a new update, but he needs one now (He's still using a very old PC from 2009). He can always sell this one on eBay/Craigslist and get a new one. That's the good thing about Apple products.
I also didn't want him to get the TrashCan Pro because it's a terrible value and 3+ years old in terms of hardware. Also has to purchase a 5k monitor. The iMac is a no brainer since it's also a monitor.