This discussion will be interesting only for those who have added to the goal the minimalism of a powerful PC.
I've been silent for too long. But now, when Intel firmly pushes tiny NUC while ignoring prospective potential of Mini ITX form factor, I'm anxious to share my thoughts of what Mini ITX system could be IF...
I think there are people who want to move to Mini ITX form factor, but they can't. They are unable to install the GPU card AND the second card they NEED. This is my trouble too. So the idea of Mini ITX improvement is to put SECOND PCIE slot (speed x1) for the second component user would like to install (to me it's advanced sound card) into Mini ITX platform. I think it is possible:
I would like to know the demand for additional PCIEx1 on Mini ITX motherboards. Please, leave your opinion here.
Some thoughts to the case composition: the CPU cooler should be used of this type with the exhaust right to the side of the case. With such cooler the PCIEx1 card would be low profile though. Best placing of the standard PSU is like in this Silverstone SG09. So it will be powerful ultra compact enthusiastic PC with dimensions like 270x350x170 mm. I didn't any sketching to determine actual size because it's useless, I've just left these thoughts here and it feels easier now.
I build Intel based systems into old Nintendo Entertainment System cases. Ive built them with i3s and a 6670 low profile vid card inside, building one now with a G530 and a 7750. Can pack a lot of power into a small form factor.
Oops! I just realized Mini-DTX is close to what I'm talking about. But still I think Mini ITX with two PCIE slots is possible and it would be smallest case for performance that Micro ATX platform has (with single GPU card).
Sorry guys! Seems I didn't manage to express my thoughts in a clear way. First post has been rewritten now.
That MSI board is very wrong, in that it doesn't align with ATX standard expansion slots, so it won't work in any ATX/mATX/ITX cases. As you pointed out earlier, it needs to extend below the board, rather than add extra items within it.
There are a couple of form factors which are defunct/barely used which do exactly what you are talking about though. Mini-DTX is 203*170mm, and is basically ITX extended to allow for a second expansion slot.
There's a whole bunch of reasons why DTX isn't likely to develop much more though, just to fire through a few -
Double slot graphics cards are incredibly common, even at near to entry level.
Using onboard graphics cuts down the need for (m)any expansion slots at all.
Modern motherboards increasingly have everything you could need (check out something like the Maximus Impact for example)
The market for this type of board would be incredibly small, even though they would fit in most modern ITX cases, as every person using a double-slot graphics card would just buy ITX.
The best argument for DTX> ITX is likely to be that the increased surface area gives you more space to put components, as ITX boards are increasingly congested. How much of that "free" space is actually useful though? It's pretty hard to say. It wouldn't entirely surprise me if a future variant of the Maximus Impact (or similar) went for effectively a Mini-DTX style board just to get some extra space though, it would potentially limit it's usage in less conventional cases like the Silverstone RVZ01 though.
Honestly though, you basically nailed it in your original post. Something like an SG09 is pretty compact, and fits a mATX board to cover your expansion needs. The bigger ITX cases like the Bitfenix Prodigy and Corsair 250D basically overlap with this market segment anyway, so in many ways the size of the motherboard is irrelevant when the size of the overall package comes into play.