Thanks for the informations.
>> old, obsolete interfaces
> Either in the form of a connector expension board (maybe a 30 pin connector that
> exports them outside the case or with a board that mounts inside the laptop
> instead of a battery pack or whatever).
I wouldn't really call them obsolete. Not all USB2something works, or you get a cheap series that uses different chips from charge to charge. And I actually did machine driving and microcontroller programming (horray for blinking LEDs!) with serial and parallel.
Such an addon board would be nice. So people who have no need for classic interfaces save the space and others can plug in a card (modularity ftw!) with native serial, parallel and whatsnot.
> CF an IDE to SATA or whatever bridge chip
This is maybe not such a problem, at least from my side. I used IDE2CF in thin clients and sometimes via these cheap adapter boards that you can attach to IDE. But for just reading CF cards a normal USB solution should be fine, too. It would have been nice to have CF directly but external USB card readers (offering all sorts of flash card formats) are available between 5 and 15 Euros (7-20 USD). This is what I normally use on the big boxes where I do not directly boot from CF.
I don't know about the difference in performance, though. When I chroot to a CF card to compile Gentoo for my small x86 machines (Geode LX, Transmeta) on my "big" Athlon II x4 there seems to be a bottleneck somewhere. Still it is faster than native compiling.
> VGA is a must and directly provided by the southbridge. If I recall correctly,
> the APU desktop chipsets support 6 displays (with DP daisy chaining).
Ah, so the SB contains some ADC. Yes, I think the Radeon Feature Matrix mentioned that newer chips might not offer native analog support anymore. But if there are affordable adapter chips that should be okay.
> There are DP to HDMI/DVI converters for everything else
I wonder about that. I was recently looking for HDMI to something else adapters and it was sheer terror. Somehow it seems the HDMI consortium forbid adapters at least in one direction.
> Addional cables will be needed in any case as connecting the HDD directly to
> laptop PCB would be very prone to breaking connectors in a moment of carelessness.
Wise decision. I guess I could still solder just cables to an interface but soldering on a board when multiple pins are to be desoldered at the same time, or even SMD stuff... uh-oh. And yes, 6 months ago I broke an external card reader and the MiniUSB cracked off the PCB and maybe the PCB even got microcracks.
It will be much better to have some pin header solution and just attach a cable to the very interface that is planted in the housing (like classic 486 where you would put every interface from pinheader to a bezel in the backside of the case).
> SATA vs. Molex
"native" SATA seems more comfortable but I saw these (quite complex by the way) SATA powers break off, while Molex seems more robust. That is, unless you manage to bend or dent the inner contact cylinders.
> That is what the external PCI-express is for!
Ex-Ternal PCI(e)? Oh, the tears of joy!
That was new to be that you had this planned. But is there also some level of protection - against dust and stuff that daily confronts a laptop from the outside?
> southbridge has 10 USB 2.0 and 4 USB 3 with blobs (have to figure out what to do here).
Yes, but that is a good number. I was glad that AMD was so nice to give even the older SB7xx chipsets a plethora of USB ports. I actually needed more and more over time.
Ehm. Blobs?
> There could be an internal USB hub and a seperate USB 3.0 controller that doesen't require blobs.
Do USB 3 controllers require blobs? I wasn't aware of that. So far I thought you did not even have specific drivers for USB controllers, regardless if the came from VIA, Renesas (NEC?), AMD, nvidia, intel or somebody else.
Blobs are evil.

> of those 10 a few will be used internally (one per each miniPCI-e slot, ...).
I am not a professional, how comes that PCIe and USB are correlated? I mean, attaching e.g. the trackpoint or touchpad via USB sounds reasonable (though the bandwidth is probably still overkill).