I don't know if you have been thinking of NTSC/PAL compatibility, but it might be good to implement that now instead of later. All you really need to change between the two is the length of vblank and overscan, plus the speed of the game. Without adjusting for speed, PAL games run at 5/6 speed, which is very noticeably different. Since you've been working on game speed, I thought I would suggest it.
I haven't implemented speed into my NyanCat project yet, but I plan on using the typical method of having an 8-bit "fractional" counter that changes your object's position when the counter rolls over. This way, you could add values that are multiples of 5 in NTSC mode, and multiples of 6 in PAL mode, and both modes would play at exactly the same speed.
Thanks for bringing that one up, because I did think about it very briefly when I was selecting palette colors. I think I can find a nice balance between the two, and I was leaning towards 50Hz as the standard difficulty speed, anyway. PAL support is definitely on the early list. It looks like the majority of it will only require different spinlock timer values and different color values (along with your list too, of course.)