> I have not dug into what the various southbridge, specialty chips contain.
> Perhaps one of them has more easily supported network or sound or something(?).

The M1575 offers Ethernet, but as Jim said, it wasn‘t available anymore when the Nemo board was designed.
You previously bemoaned that with a PCI NIC connected to the SB600, the network traffic would have to go through the southbridge. How would that be different with a southbridge incorporating the Ethernet controller? And what‘s wrong with the SB600‘s audio?

> If for example one was connected to one or two PCIe lanes it might have provided
> two more PCI slots for lower demanding add in cards

As said, all of the PA6T’s PCIe lanes are in use, so dedicating some to PCI would have impacted the PCIe capability. Also, two more PCI slots would have required the space of at least two of the PCIe slots, resulting in only few PCIe expansion options. I don‘t think that would have been welcomed by potential customers. And I believe two legacy PCI slots should suffice.
Btw, a southbridge chip is extreme overkill just for providing PCI slots. A mere PCIe-to-PCI bridge chip (as used on Cyrus Plus) would do.

> Or also connect the XMOS chip to the second 'southbridge' instead of the localbus.

A-Eon seemed very fond of Xena’s tight integration with the PA6T.