Two things that I would ask about when considering this:
1) What product is Vega aimed at?
2) What product is Zen aimed at?
If we consider that the iMac really is their main desktop line for the mainstream user market, starting to eek into the professional market, things get a little weird. Zen in a Mini or Mac Pro to bring a lot of cores to the Mini as a microserver, or bring prices on the Mac Pro down by not relying on Xeons? I could see that. Less so for the iMac when single-thread perf is still important for more mainstream apps (i.e. not Lightroom/Photoshop or Final Cut/After Effects).
My first question for you would be:
1. To whom is the 27 5K iMac really aimed at?
When it was introduced in 2014, it wasn't as the next generation of casual computing, it was as a pro machine that offered a world class screen that even a truly "pro" desktop couldn't match. It was aimed at photographers, video editors, and others for whom the extra resolution would really and truly matter.
While it's certainly true that as the 5K screen has migrated down into the lower end 27" models, Apple has created a 5K iMac that is really more for the casual user with too much discretionary income and not enough common sense (given the poor performance of the lowest end model), I don't think your characterization of the 5K iMac as being aimed at "more mainstream apps" (for example?) is necessarily correct.
While I do agree with you that Apple does, and should value single threaded performance in the iMac (and across their product lineup) (as they should), all the indications are that, as long as AMD can deliver on clock speed, we're looking at at least Haswell levels of single core performance which is more than adequate for even poorly threaded apps. If Apple can have haswell levels of single core performance, AND more cores (and possibly even a lower price), I think that would be a very attractive proposition for both Apple AND consumers.
Given that the 5K iMac is a machine serving many markets, including pro and semi-pro, I think there is definitely room for (at least a high end model) supporting Zen/Vega/HSA. One of the things that could make this combination so attractive is that an APU could dramatically reduce the required board space and TDP requirements for a high end CPU and GPU, giving Apple more room for cooling, new features or, god forbid, additional thinnovation.
Vega is also interesting, but Apple historically hasn't really been putting focus on anything beyond general compute for GPU performance. And with the Vega aimed at being the super-high end of AMD's line, putting it in an iMac would also be a little weird. Apple seems fit to keep a slow HDD so they can put in a 5K display on the iMac. I don't see them doing a redesign so they can put a ~600$ MSRP GPU into it as a BTO option.
I would disagree with the above assertion. Over the past five years, Apple has shown that they have come to care about graphics in the iMac. The 680MX in 2012 was a revolution for an all in one, and featured performance within striking distance of some of Nvidia's top end desktop chips, and while the R9 295MX and R9 395MX have been less impressive in this regard, they still offer solid performance within the TDP (and manufacturer) Apple has had to work with. That and the mobile variants of these cards do often have MSRPs over ~$600, although I don't think anyone believes Apple actually pays that much.
Furthermore, while i didn't mention this in the initial post, lower end APU's could of course utilize a Zen/Polaris APU and forgo HSA to keep costs down.
Whats important to Apple (IMO) is how much performance they can get into a given design, and what kind of differentiation they can offer the market. I think AMD's APU's offer Apple interesting options on this front.
Things like HSA would be interesting, and something Apple might go for if it offered an advantage. But that looks more interesting in the Mini than the iMac right now. Considering one of the things Apple is doing is making the fastest Quad Core Skylake chips Intel offers available to customers.
My own bets are that Intel is here to stay in the iMac for the time being. AMD has a couple of good niches, and Zen offers some options in those niches, but I don't think the iMac aligns with those niches all that well. And I don't think ARM is even equal to AMD. I agree that if there is a switch, AMD is a better bet than ARM, but I don't think AMD is a great bet either.
That said, if Zen could make the Mac Pro a bit cheaper again while being generous with the cores, and offer up some serious GPU hardware in the form of Vega... I might just consider buying a Mac Pro again instead of an iMac. But I would miss that sweet single-thread perf the i7 line has.
I'll agree with you, (and most people in this thread), the probability, particularly for this event this week, is probably pretty slim, and I'm not expecting it so much as just throwing it out there to start discussion and see if anyone else sees what I see.
One more quick thing. In responding to your post, I thought about how Apple initially released the 5K screen only on the high end iMac and kept the old (at the time 2013) iMacs around as a lower cost option.
While I've been pretty skeptical of the iMac Pro idea, I now realize the precedent for a "high end"/
"pro" iMac which compliments and doesn't replace the current models, is certainly there. While I certainly hope it doesn't replace the Mac Pro, I'm hopeful that perhaps our resident analyst who said "probably iMac not ready yet" just wasn't looking in the right places