While the access panel would be something that would help a lot for example with replacing CPU, cooler and m.2 ssd, the issue is we are saving space everywhere we can including under motherboard, and when we designed potential locking mechanism for that access panel that could work, it was something that might be hard to manufacture and pose risk of being unreliable and prone to damage.
You have to remember that the area under motherboard has defined restrictions in ATX form factor specification, and thus if we want to be compliant, we need to obey those. That's one of the reasons why the round feet for horizontal orientation are plastic riveted feet and not ones that are mounted with a screw because that one feet that goes under the motherboard would need to have a bushing with thread inserted there and it's against the spec to intrude with conductive elements in that area.
Figuring around something that would fit - be a really, really slim, like 1.5 mm thick mechanism on the inside, is really hard. And on top of that figuring out a way to design the hole in a way it doesn't degrade the aesthetics, while it is still useful to access M.2 and cooler mountings, without compromising the case rigidity is also hard. Finally most of the people voted against the concept when we had the poll for it.
As for support for 2 x 2.5" drives, it will still be there, but if we want to support 2.5 slot cards properly, we need a bit of leeway for them, so the rail with riser mount and 2.5" drives will go slightly up, which means supporting slimmer drives than before. In 2.0 we could still fit 10.5mm thick drives (there are 15mm, 12.5mm, 10.5mm 9.5mm and 7.0mm SSDs) but it might mean dropping down 2.5" support to 9.5 mm, which in essence means only SSDs and 1TB drives, but the latter depends on the drive construction. Anyway whether we drop this is also dependant on whether the case will remain in exact same dimensions or whether it'll slightly (1 or 2 mm) grow in thickness.
