would it have been technically possible to add..

[Tatsujin]

..some additional hardware specs in to a systemcard to upgrade the DUO in terms of real 2 (or more) layer scrolling or other hardware effex?

[Bonknuts]

 Audio upgrades are possible via the cart port, but the only other way to get any extra BG layer would be the 32X method. Or something similar. Not a very elegant solution. You'd have video cords and cables running out of the from cartridge slot area.
 
The CD base addon could have easily added an additional BG layer - similar to the SGX setup. They could have even added a new VCE (bigger palette), etc since all the internal hardware is directly accessible via that external back bus. Of all the game consoles, the PCE back bus was incredible and such a waste that they never put it to good use. No other system gave every internal pin out to a bus such as the PCE did.

[Black Tiger]

The CD base addon could have easily added an additional BG layer - similar to the SGX setup. They could have even added a new VCE (bigger palette), etc since all the internal hardware is directly accessible via that external back bus. Of all the game consoles, the PCE back bus was incredible and such a waste that they never put it to good use. No other system gave every internal pin out to a bus such as the PCE did.

So basically, any kind of upgrade could be done through the EXT bus?

[Game and Watch Kirby]

Well, I can say that it would be highly possible, and definitely with the right programming. It may also require a change in hardware to make the enhancement able to perform, so the PC Engine would be able to better handle the instructions, such as the similar microchip that the Sega Mega Drive/Genesis uses for multiple layers of parallax scrolling (if one is used to do so).

[Bonknuts]

So basically, any kind of upgrade could be done through the EXT bus?

 Yes. Any. The external bus is incredible on the PCE, as it provides every pinout inside the system available externally. Such wasted potential 

[ceti alpha]

So basically, any kind of upgrade could be done through the EXT bus?

 Yes. Any. The external bus is incredible on the PCE, as it provides every pinout inside the system available externally. Such wasted potential 

Yes, very sad. But, that would have probably have increased the price of the already expensive CDROM2 attachment.

[ccovell]

I think it's safe to say that NEC was a very marketing-driven company, as opposed to an engineer-driven company.  They kept on releasing gobs of plastic that were rather unnecessary, and never did development on a next-generation PCE until much too late.