ASRock has put out some more recent "mining" targeted boards, but I don't pay much attention to them as I don't build riser rigs and couldn't care less how many PCI-E 1x slots a board has.
Just a curiosity: by "I don't build riser rigs" do you mean you only connect gpus to full pci-e x16 (at least electrycally) slots?
So do you only get 2-3 gpus per rig / motherboard?