I was going to recommend DIN 41612, but you already know about them. Back in the day, VME and VXI bus backplanes routinely pushed 50 MHz signals. What is your worst-case pin count for a daughter card? Also, worst-case current?

Another option is the PCI bus parts used in IBM PC clones. The bus started out at 33 MHz, but grew to 133 MHz.

https://en.wikipedia.org/wiki/PCI-X

I don't recall if PCI-X and PCI Express used the same connector types, but they both supported 100 MHZ-plus.

I was going to recommend DIN 41612, but you already know about them. Back in the day, VME and VXI bus backplanes routinely pushed 50 MHz signals. What is your worst-case pin count for a daughter card? Also, worst-case current?

Another option is the PCI bus parts used in IBM PC clones. The bus started out at 33 MHz, but grew to 133 MHz.

https://en.wikipedia.org/wiki/PCI-X

I don't recall if PCI-X and PCI Express used the same connector types, but they both supported 100 MHZ-plus.

Unless you need a ton of pins, I would stay away from cPCI (money) and VPX (VITA47) (more money).