This is something tomatheous and I (and a few others as well) had talked about on IRC awhile back, but never really did anything with. So, I'll outline some of the ideas that we came up with as well as some of my own that I considered afterwards.
THE PLAN:
A new system card, revision 4.0. Designed to play all existing PC Engine/TurboGrafx CDROM software as well as new software built specifically for the new card.
WHAT IT WOULD HAVE:
Switchable rev 1.0, rev 3.0, and Games Express BIOS code + normal RAM
Full ACD support
New 4.0 BIOS, supporting the modern CDROM standard
Onboard 4-channel Vorbis decoder with 8MB buffer and mixer (would all but replace the built-in ADPCM circuit)
tomatheous also suggested an onboard coprocessor, possibly an NEC V810, but this would probably be very difficult to do
Like the original cards, two versions would be available; one for the normal hardware and one for the Duo.
Most of the card would likely be handled with an Actel flash-based FPGA chip.
Any suggestions, comments, criticisms, cash offers for development, etc. are welcome.