great customer relation!
I think there are 3 Points to consider: the type of the connectors, the type of the trays and the combinability.
As for the connectors I'd propose to use sff-8643 only for SATA. The reason for this is that when you have sata only mainboard or controller, you can use a sff-8643-sata-breakout cable. That is quite standard and would allow to use one type of backplane for 2.5 inch sas/sata trays. Nonetheless most Mainboards and Controllers already use sff-8643 interfaces.
As for nvme drives OCulink connectors are also standard, so no change is required here.
For the trays I would prefer m.2 NVMe over U.2 (m.2 is cheaper). For SATA my requirement would be that all lanes of an sff-8643 connector should be usable - which means that 6 m.2 sata ssd for two connectors is a waste of 2 lanes. I would prefer to have 4 2.5 Inch sata ssd, but I may also have an application for 8 m.2 sata ssd (enough space seems to be there). Atm. a 2.5 Inch sata SSD for datacenter (Kingston DC450R) is much cheaper than any m.2 sata datacenter ssd, so that is my preference.
And as for combinability, I'd prefer to have detachable modules that can be rearranged.
* 2 x m.2 NVMe module
* 2 x U.2 NVMe module
* 4 x 2.5 SATA module
* 8 x m.2 SATA module
* blind panel module (really important)
An USB/CardReader module may also be of some use or a fan control unit.
As packaging I'd like to see packages with one ssd module and one blind panel module. If you need two ssd modules, you have two blind Panel modules as spare - a thing I had use of many times (but did not have one).