Whatever the benefits of Broadwell over Haswell, the quad core Broadwell that was announced today will probably be an upgrade over the Haswell. It will also not required a redesign of the current rMBP. The Skylake chip will probably require a remake of the notebook and I'm betting that it will be 2nd quarter 2016 before it happens so that both the 13" and the 15" rMBP can benefit from the makeover. Just a guess on my part
You're most likely correct that the redesign of the MBP's will come Super late Q1 (Think end of March) or Q2 2016. It isn't that Skylake will require a redesign rather give Apple an excuse and a reason to do so.
It would be highly unlikely that they don't take the opportunity of the new architecture and TB3/USB-C port technologies to shave off several millimeters of thickness, probably ending up somewhere around 13.5 - 14mm or so. Active cooling will be the limiting factor here as far as thinness goes. MagSafe will be gone. It is questionable whether they will leave legacy ports on there or not - knowing Apple, I am guessing not. I'd rank it as "probable" that you will not see a mDP, USB-A, HDMI or SDXC port on these at all, much as lots and lots of you don't want to hear that.
We're most likely going to see two TB3 enabled USB-C ports, one or both of which will be designated as the "power/charging port(s)", and maybe two additional ordinary USB-C ports. It will probably
NOT be a tapered design ala MBA or rMB, rather a refinement/evolution of the current "flatbed" design they have now. It will probably get slightly narrower as well, even though the screen size might, just might, get a 1" bump across the board, see below.
Thinness won't only be achieved by shrinking the bottom part of the chassis, but also bringing the rMB display technologies to the Pro line. On the 15" (Maybe 16"?? - side note hunch, 12" iPad Pro, 12" & 14" rMB's, 14" & 16" rMBP's, 13" MBA stays on unupgraded staying on Broadwell, 13" cMBP stays on also unupgraded, 11" MBA killed) you will see a low TDP dGPU, don't expect a super high powered monster - you see what they have chosen to put in the 5K iMac, so let's not be unrealistic here.
Greater than 50/50 chance in my book that the rMB keyboard gets carried into the redesign of the MBP's as well - Apple engineers are personally contacting almost all rMB owners reporting a problem with the keyboard, they are extremely keen on it for some reason, I think this is why - and there you have it, the realistic expectations for the redesigned 2016 MBP's.
It probably isn't going to jive well at all with those calling for a chassis thicker and bigger than the MBP's are now, or envisioning the unlikely return of an Ethernet port onboard, but that stuff was never going to happen in a million years, it just isn't Apple's MO.
Having had an 11" MBA for the past 4 years as my primary professional machine (finance, accounting & management consultant, not audio, photo or video to be clear) and now just waiting for my rMB to arrive, it is already second nature for me to have a couple of adapters in my laptop case, for those occasional times they are needed. And that, I think, is what Apple sees as the point.
Think about a 5K Apple cinema display (or 3rd party, whatever) that has all of those legacy ports onboard, and connects them all to the laptop at full speed
AND provides said laptop with power
AND can daisychain to that external RAID array for super mass storage,
AND does all this with one cable. No spiderweb adapter mess at all. Simple, elegant, super fast and efficient. That's pretty much Apple in a nutshell.