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The question has been asked pretty much since the start of the project: "Since there's already three slots, why not just add another to make it microATX?"
It does not work that way.
To begin with, mATX is 244x244mm, while mini-ITX is 170x170mm. If you were just to add an extra slot and increase the height by 20mm, a mATX motherboard would not fit. In fact, it would extend into the space currently occupied by the power supply. In order for it to fit, you would need to increase the length of the case by 74mm - from the current 328mm, up to 402mm.
Secondly, mini-ITX is limited to 84W CPUs and a single GPU of up to around 300W. The current 450W power supplies are adequate for powering such a system. However, mATX opens up the possibility of using 150W CPUs and dual 300W GPUs. This is an effective doubling of power consumption, for which even a 600W SFX is insufficient. Therefore, in order not to limit the capability of the form factor, an ATX power supply should be the default option for mATX. This means a further size increase, to accomodate not only the larger dimensions of the power supply itself, but more and longer cables compared to SFX.
Thirdly, the airflow scheme in the M1 is inadequate for dealing with the extra heat produced by a second GPU and more power-hungry CPU. To provide sufficient cooling, the entire layout would need to be re-engineered, and in all likelihood would result in more or larger fans and an overall size increase.
I've thought about this a lot, and in my opinion, it's not possible to meet the cooling and power requirements of a high performance microATX system in a package significantly smaller than what Silverstone has achieved with the SG09/SG10. You can see what it took to achieve that: fans in nearly every available space, resulting in a vent-heavy design, which contributes to its homely appearance. And even at 23 liters (10L more than the M1), there's no space for anything more than a single 120mm radiator.
Supporting microATX has a lot of implications for the rest of the system, which, as you can see, go far beyond simply "adding another slot."
It does not work that way.
To begin with, mATX is 244x244mm, while mini-ITX is 170x170mm. If you were just to add an extra slot and increase the height by 20mm, a mATX motherboard would not fit. In fact, it would extend into the space currently occupied by the power supply. In order for it to fit, you would need to increase the length of the case by 74mm - from the current 328mm, up to 402mm.
Secondly, mini-ITX is limited to 84W CPUs and a single GPU of up to around 300W. The current 450W power supplies are adequate for powering such a system. However, mATX opens up the possibility of using 150W CPUs and dual 300W GPUs. This is an effective doubling of power consumption, for which even a 600W SFX is insufficient. Therefore, in order not to limit the capability of the form factor, an ATX power supply should be the default option for mATX. This means a further size increase, to accomodate not only the larger dimensions of the power supply itself, but more and longer cables compared to SFX.
Thirdly, the airflow scheme in the M1 is inadequate for dealing with the extra heat produced by a second GPU and more power-hungry CPU. To provide sufficient cooling, the entire layout would need to be re-engineered, and in all likelihood would result in more or larger fans and an overall size increase.
I've thought about this a lot, and in my opinion, it's not possible to meet the cooling and power requirements of a high performance microATX system in a package significantly smaller than what Silverstone has achieved with the SG09/SG10. You can see what it took to achieve that: fans in nearly every available space, resulting in a vent-heavy design, which contributes to its homely appearance. And even at 23 liters (10L more than the M1), there's no space for anything more than a single 120mm radiator.
Supporting microATX has a lot of implications for the rest of the system, which, as you can see, go far beyond simply "adding another slot."
