I was trying to point out that AGP wouldn't put you in a much better position than PCI (except for the 2nd hand market).
OTOH, a AGP to PCI adapter can be fairly simple if the firmware/chipset support such and either use single slot PCI or two (bridged) busses - but that would lower flexibility or raise costs.
PCIe is a technology that is not easily implemented in CPLD and would require substantial additional logic/codecs, so in small volume markets it's no choice, unfortunately.
Additionally, another major problem of a niche market is the availability of drivers, so supporting current hardware is not realistic anyway. When looking at price / performance trade-offs, PCI is a rather reasonable choice as long as there's any hardware available.