They all use the same socket. I wasn't very clear.
To be even clearer they DON'T use the same socket. The U Series Haswell (dual core) used socket FCBGA1168 while the HQ series Haswell (quad core) used FCBGA1364. For Skylake we have FCBGA1356 on the dual core U series and FCBGA1440 on the quad core HQ series.
For Kaby Lake we only have the 15w U series to go on but the Intel ARK site says that uses FCBGA1356 as well (unsurprisingly the same socket as for Skylake). I would say when HQ series Kaby Lake CPUs come along they will be FCBGA1440 as with the Skylake ones.
If Apple's budget for engineering the Mac Mini remains the same they'll have to choose between dual core U and quad core HQ CPUs. Given that Kaby Lake HQ cpus don't even exist at the moment (only Skylake out at the moment) and that the only U series dual core Kaby Lake cpus are the 15w ones (the 28w ones aren't out till early next year) the easy betting is for the Mac Mini to use the 15w Macbook Air Kaby Lake CPU across the board.
You might then find that the HD 620 graphics on the Kaby Lake 15w CPU actually outperforming Haswell Iris Graphics 5100 used in the 2014 Mac Mini. If you hand the 2016 Mac Mini a decent cooling solution and fast enough DDR4 RAM it might well be able to run rings around a system that uses almost twice the power for sustained periods.
Here's the link to
HD 620 (probably the next Macbook Air 2016), and here's the link to
Iris Graphics 5100 (2013 Macbook Pro 13"). And
Iris Graphics 6100 (i.e. 2015 Macbook Pro 13")
Considering the fact that the clock speeds between the processors are now roughly the same, with CPU benchmarks probably favouring the Kaby Lake architecture, and then perhaps make sure that every 2016 Mac Mini comes with SSD to speed up IO, and Apple being able to utilise economies of scale by buying the same CPU in bulk for the Macbook Air and you have a win-win-win situation for Phil Schiller on Thursday.
Is this the time to suggest that the recent mentions of the Macbook Air in multiple rumours could be because Apple have decided the Kaby Lake 15w chips are here, and are an adequate upgrade to go into a retina laptop that will supplant the Macbook Pro 13"? The graphics would get a nice boost from the use of fast DDR4 memory (which is a slightly bigger power drain but battery capacity would increase if the 15w CPUs went into a Pro style case), and we might even see a price reduction.
Realistically, the range looks simpler if we had:
5w Kaby Lake CPU - 12" Macbook (with significant price cut or lower entry level)
15w Kaby Lake CPU - 14" Macbook Air with Retina (or call it 14" Macbook Pro)
45w Skylake CPU - 16" Macbook Pro (with AMD Polaris GPU)
Give it 2 models of each.
And then release a Mac Mini using that 15w Kaby Lake CPU, keep the same form factor if 2.5" hard drives for Fusion are still a factor - otherwise consider making an all SSD model smaller.
And assuming the iMac is waiting for Kaby Lake desktop CPUs and high performance Vega GPU, and a 27" 5k Retina Cinema Display is delayed could that leave us waiting on a refreshed Mac Pro which may also be also waiting on Vega GPU but already planned to use Xeon E5v4 Broadwell EP CPU?
All of these models would have USB-C/Thunderbolt 3.