if for servers, maybe xeon phi is not selling so hot.
if u mean for the consumer market, what would be the application? I kind of toyed with the idea of having a dual OS system through KVM switch, so that I wouldn't need to run a virtual box (and the considerable slowdown and vbox setup issues) to have win or linux run at the same time. So for a similar purpose it might find a few niche users.
I don't think there are many applications in the consumer space where general purpose add-in cards make sense, it is niche even in the enterprise since specialized hardware is more performant and efficient at the tasks they are intended for.
There would also be the issue of bandwidth and latency, multi socket motherboards already fulfill niche areas that need more general purpose compute without the limitations of a x16 PCIe link.
Now if we are talking multiple independent systems in a unified enclosure, that is also already served by products such as IBM Blade servers.
So any valid use case for a PCIe x86 product would be *very* niche and is IMO a solution looking for a problem.