MSX + Arduino Gameduino shield and ZX SPI

yes Gameduino is like a vdp. It takes commands sent in SPI protocol. It is designed for arduino, but you can connect it to anything that can do SPI. It also does sound, and also can be used as a co-processor (has a forth based processor in fpga). There is also a newer and more capable Gamediino 2 as well.

Yes, for sure Arduino is connectable to MSX... I ie. use it regularly to translate PS/2 mouse communication to MSX mouse communication. As experiment I've even downloaded few games trough joystick port using Arduino to translate data from USB to MSX. Initial loader was MSX-BASIC oneliner that was short enough to remember and then it was replaced with ML loader for faster data transfer before loading actual ROM-game.

For attaching Arduino trough cartridge slot is a much harder task. This kind of SPI interface could be nice for that kind of purpose as well. I've seen some experiments about much more simple alternatives, but they still need extra components and tend to slow down the whole machine during memory / IO access... Is that a real problem depends of what you are trying to do.

I think that today there could be small market for some modern general purpose SPI, I2C & GPIO interface for MSX that could be easily controlled. (hint, hint) I know that you can do bit-banging without extra hardware, but that slows things down quite a bit and requires extra code layer. Also MSX has only two pins / joystick port that really can be used as GPIO and even those are pain in the butt to control trough PSG especially when BIOS is messing in the same soup as well... This became painfully clear for me when I attached PS/2 keyboard trough joystick port... How ever I was able to use MSX-BASIC & MSX-DOS with it

cool, I'm making new hardware with this chip: ft813 and I'll put it in msx.
the advantages over v9990 are many, especially the price.

interesting, also nice chip choice...
man it is such a powerful little chip:

skimming through the datasheet, I came to a question, what mcu are you going to use to drive the ft813? or you would some how drive it from the msx bus directly?