There is this old web site that shows a design of a H8 CPU/microcontroller cartridge design for MSX. (All in Japanese, but there are schematics and pictures there)
www.nana.or.jp/~hiramats/h8/msxh8.html
And there is another web site that shows some applications for it, like 3D renderer and some graphics effects. (It doesn't show properly on most web sites, but I downloaded the mpg data from there.)
You hear some people talk about this sort of cartridge design often, but this one is a working design. It would be cool to build it, but the H8 CPU doesn't seem to be easy to get hold of outside of Japan.
Replacing that with something more readily available like Atmega microcontrollers would be an idea, but they don't support external RAM (internal is between 1KB to 8KB I think. And for this H8 design they got 1MB....)
