Sorry for the thread necromancy, but I've been thinking about this sort of stuff recently, and visualized a very skeezy solution.
1x Amfeltec hextuple M.2 Squid host adapter card (I like to call it the Beast Squid)
SQUID Carrier Board™ Family PCI Express Gen 3 Carrier Board for 6 M.2 or NGFSS (NF1) PCIe SSD modules - Amfeltec Corporation - innovative hardware products for all kind of computers, computer peripherals, communications equipment, and similar electronic products.
6x M.2 extenders for clearance away from the Squid (double sided means less clearance to the next PCIe slot, so most M.2 cable adapters won't fit, barring maybe that Intel M.2 to U.2 cable)
NGFF M.2 PCIe M-Key extension card with 20CM cable - Sintech adapter card and test tool
6x M.2 to SFF-8643 adapter cards (mostly offered with the SFF-8643 connector facing the M.2 connector like the Startech one, though there are some others that offer the connector pointing out from the board or away from the M.2 end)
M.2 M-Key Mini SAS HD SFF-8643 Adapter
https://www.amazon.com/dp/B018GHPU6W
Your choice of how to cable from SFF-8643 to either SFF-8643 or to U.2 directly to reach your SSD's.
For example, an IcyDock quad NVMe cage for an ODD bay hosting 4x2.5 U.2, uses 4x SFF-8643 on the backside
MB699VP-B_ToughArmor Series_U.2/M.2 SSD CAGES_ICY DOCK manufacturer Removable enclosure, Screwless hard drive enclosure, SAS SATA Mobile Rack, DVR Surveillance Recording, Video Audio Editing, SATA portable hard drive enclosure
IcyDock is also contemplating new NVMe bay cage concepts
CP055_NEW CONCEPT PRODUCTS_CONCEPT PRODUCTS_ICY DOCK manufacturer Removable enclosure, Screwless hard drive enclosure, SAS SATA Mobile Rack, DVR Surveillance Recording, Video Audio Editing, SATA portable hard drive enclosure
CP067_CONCEPT PRODUCTS_ICY DOCK manufacturer Removable enclosure, Screwless hard drive enclosure, SAS SATA Mobile Rack, DVR Surveillance Recording, Video Audio Editing, SATA portable hard drive enclosure
CP081_ALL CONCEPT PRODUCT_CONCEPT PRODUCTS_ICY DOCK manufacturer Removable enclosure, Screwless hard drive enclosure, SAS SATA Mobile Rack, DVR Surveillance Recording, Video Audio Editing, SATA portable hard drive enclosure
Since the Squid comes in both x8 and x16 edge connector flavors, using an x8 connector would allow you to use the PLX switch to oversubscribe/fan out your PCIe connectivity to up to 6 downstream devices (6x4=24 PCIe lanes total).
Is this semi-reasonable, if you can handle being over-subscribed on the host PCIe interface?
-----
To be honest, it would be very interesting for Amfeltec or someone else to build a highly oversubscribed PLX based host card. PLX chips max out at 96 lanes, which would suggest one could build a Squid card with an x16 edge connector feeding 20 SFF-8643 ports (bonus points if they can do that in a HHHL sized card).
Another oddball card that would be a nice to have, would be a half height to full height PCIe to PCIe riser adapter card, with a PLX chip and an M.2 slot spliced in. Something based on Startech's PCI to PCIe half height adapter as a design pattern
PCI to PCI Express Adapter - PCI/PCIe Adapter Interface Card | StarTech.com
or their x1 to x16 adapter
PCIe x1 to PCIe x16 Slot Extension Adapter | StarTech.com
this guy does custom PCIe bifurication risers, and illustrates the concept, though I would try to move the M.2 area out beyond the PCIe connector area to reduce the stack height
PCIE Bifurcation
but ideally with the Amfeltec adaptable edge connector from their Squid cards, so that bottom connector could switch between x4/x8/x16, with a top being an x16 connector for the raised half height card. Why you might ask? There are plenty of PCIe cards that are half height that come with both half and full height brackets (quad NIC's and low end GPU'S for example). Such a riser card would effectively allow you to expand an occupied full height slot populated with a half height card. The M.2 slot would be good for a boot drive or similar.
-----
The custom PCIe bifurication riser card guy now has a web shop
About Us
and appears to be dabbling in custom PLX expansion riser cards
PCIE Bifurcation