It's funny you mention the TMS9918. I was looking for a good place to link my friend's drop in replacement for the 9918 he made for his TI 99/4A.

http://codehackcreate.com/archives/30

"F18A stands for “FPGA TMS9918A” (hence the project’s title) and will be 100% hardware and pin compatible replacement for a TMS9918A VDP (video display processor)."