I don't know if this applies to your current design, but when I saw the thread I envisioned my M8 with only one side. Going by that as a guideline, there's only a few things I'd really change for a single wide.
1) On the M8 there is a wiring access hole on the PSU side that allows you to run wires through the interior roof to under the top/extended top. Adding a second hole to the motherboard side would have been great for cleaner wiring of a mobo side rad up top. Even if we're talking about a single wide case, if behind the mobo tray is 2", a hole on both sides would help with wiring cleanliness.
2) If this single wide is intended to have an extended top option, a cut for fittings to pass through the roof down into the mobo area would be great. This was a major reason I opted to mount rads inside the case instead of under an extended top. I didn't want to fuss with cutting fitting access holes. The holes could be blanked if not needed. It's just that imo if you're building a single wide, real estate is a concern for liquid coolers that are in the market for a single wide tower. Having the option to mount the rad above the roof and not have to cut out fitting access holes would be a nice bonus. It also gives the possibility of saving the top 5.25 bay.
3) This is a very minor quirk, but a suggestion for all cases. One mod I had planned for my M8 is a fan frame cover for the flex-bay fan mounts. For me personally it was a bit of an eyesore on my beautiful case to see the LED fans on the PSU side only show the fan blades (think just a circle of light) but on the mobo side, the flex bays show the illuminated fan frame as well. This only really applies to clear frame LED fans, but it's just something I would change. Again, very minor detail.
Just my few cents. Looking forward to seeing the single wide