Next generation should be 100% compatible with A4000 and run Workbench 3.9. It should have at least one videoslot and one Zorro slot.
It should not run on PPC but on a faster 68k CPU so that software should be able to run without recompile. Off course it should have PCIe and SATA etc etc, and it should be available in Mini-ITX/Micro-ATX
I think it is important that the Amiga chipset is on the motherboard and not in software.