It has been tried in each generation of motherboard design but in an era where GPUs had a custom motherboard slot that normal cards could not occupy it made a sort of sense. And I know there have been times where the northbridge could not saturate as many PCIe devices as one might have motherboard slots. So even leaving the slot intended for GPUs empty or populated with a daughter card might be leaving performance on the table. But I suspect a riser card would fit handily into a 16x slot without blocking more than one or two 2x slots.