2017 Predictions:
Mac mini - Apple changes to the low-power "m" CPU in order to eliminate the fan. Might possibly make the external case thinner, as well. Why? Ask Jony Ive's therapist. Result: A more "mini" mini (watch Apple's ads for this slogan). Power requirements are down but with no significant improvement in performance. No upgrade possibilities at all as RAM and whatever you order for an SSD will be soldered down to the motherboard. Now that Apple's learned how -not- to leave any money on the table, expect nothing in the way of a machine you'd want to own. This is not Apple's version of a NUC or BRIX; rather, this is Apple's version of the "Compute Stick". If you approach it from that direction, it may make some technical sense but the pricing will be laughable.
iMac - Still 21' and 27" as the economies of scale are there. Probably will be a low-end 21" unit with the low-power "m" CPU so the size or number of fans may be reduced due to the lower power requirements. Motherboard video only. Again, this is not a unit any reader of MacRumors would want to put on his/her own desk but it might have some leverage in schools that haven't yet succumbed to the "1 child/1 computer" siren song; it might also be okay for your old Uncle Harold in Peoria if you set it for a lower resolution for his failing eyesight.
As for the "better/best" models in the iMac lineup, whatever AMD provides will have to be throttled back in order to prevent the video card from frying inside the tight confines of the iMac case. Apple has made its bed with tiny, frugal fans and this won't change until (unless) Apple sh-tcans that ridiculously thin case that serves no purpose other than to hobble future development of this model. However, don't look for this to happen as long as Jony Ive is in Apple's employ.
Mac Pro - Don't look for a new form-factor here; again, Apple's engineers are making products to Jony Ive's specs without regard for the wishes of its most loyal customers. Faster CPU? Possibly. Faster GPU? Available at $$$. Cheaper? Probably not although if you do see some dramatic reduction in price, that's your clue that the days of the Mac Pro are definitely numbered.
If the Mac Pro is discontinued, that's the end of the Macintosh as we know it. Look for another CPU shift as Mac-on-ARM becomes the norm. It's a lot easier--both in technical and political terms--to switch when there isn't that last one Mac requiring Xeons.
What of the MacBook and MBP models? Nothing substantive will be changing except the price as Apple realizes it really screwed the pooch. Maybe by November of 2017 you'll see something you might consider a major improvement (beside price). But I really don't think this will happen. Again, Jony Ive's obsession with thinness has ruined the MBP's keyboard (and I invite those who have not laid hands upon it to do so - it's a disaster). No way will the MBP become thicker again (even though 2 mm or so would permit a decent keyboard to be installed and let the unit run cooler).
So, what would Steve do?
1. Fire Jony Ive; he outlived his usefulness and essentially became Apple's worst enemy by designing stage prop computers.
2. Have Apple's engineers make a Mac mini with expansion capabilities (RAM, HD). Add a third Thunderbolt port; after all, isn't that the future?
3. Have Apple's software engineers craft a macOS extension that permits multiple Mac minis, connected via Thunderbolt, to cooperatively process (similar to what they've already done with FCP Server but not over Ethernet).
4. Apple's "Pro" users could be served by an (optional$$$) Apple-designed Thunderbolt expansion chassis for whatever awesome double-wide video card can be made to work with macOS. In addition, these Pro users may purchase as many minis as they need. Let's imagine four Mac minis each with 32GB of RAM and a fast SSD interconnected via Thunderbolt with one or more minis in the chain connecting to expansion boxes with great video cards (or whatever else can fit in such a chassis). Four minis make a machine with four quad-core i7's, 128GB of RAM, and a fast video card. Or maybe that video card isn't even needed; after all, encoding video takes place on the i7 CPU so, with four of them divvying the work...well, you get the idea.
So, if Steve made this happen, we'd be able to reduce the Mac product line (which Steve always loved doing).
But Steve isn't there and the bean-counter king with his bulemic court jester are foisting off invisible clothing of magic cloth that only the "Mac faithful" can even see.
I call BS.