There seem to be some technical limitations that Matthew has already highlighted which get in the way of simply extending the number of bits assigned to palette vs number of bits assigned to the pattern. I've pasted the relevant reply below:
I'm not disputing that having a fully and freely selectable palette of 16 colors anywhere on the screen would greatly help you , I'm convinced it is a whole lot more comfortable for you to work with less limitations and not more.
but I wouldn't go so far as to say this is how all - or even most - pixel artists feel. If I can make an analogy from my perspective as a coder, I actually enjoy the period-correct limitations quite a bit. I have very little interest in programming modern machines these days because there are so many tools out there that make it so much easier to create impressive looking results. The limitations of the TI99/4A provide challenges that are fun to try and overcome, and I have a much bigger sense of accomplishment whenever I've managed to do something that works around the limitations of the hardware.
I wholeheartedly and 100% understand that not everyone feels that way, and I'm absolutely not judging those who don't. Just wanted to point out that not everyone has the same reasons for enjoying this retro computing thing.
Having said that, just like everyone else I have some ideas regarding new features for the F18Amk2. I've always been a big fan of the way the Sega Master System handled it's graphics. The SMS VDP is a direct descendant of the tms9918a (it supports all of the tms9918a's modes), and does in fact add a 4bpp mode. In that mode, you have access to one of two palettes of 16 colors per tile (sprites are forced to use the second palette, no choice there). The mode is called "mode 4" in the master system world, and would be a nice complement to ECM1..3 in my opinion.
But I fully understand that my opinion is just that... one of many opinions. I would prefer this way of extending the F18A, some other people might be more in favor of extensions that follow the programming model of the Yamaha 99x8 chips a bit more closely, while yet other people would like a more CGA/EGA/VGA based approach. Can't please everyone, and I for one am pretty excited to explore whatever Matthew comes up with eventually.
I'm note sure if you already discovered this, but /if/ you don't mind losing half of the horizontal resolution, the current F18A actually does support a full 4bpp bitmap layer that can span the entire screen.
Alternatively, as Tursi has pointed out, the current F18A in ECM3 mode actually does give you more than 16 colors on screen (up to 63, like the Sega Genesis), and is only limited to 15 colors per 8x8 tile. And that is without resorting to scanline tricks. I would love to see if we can take an image of yours and use the current F18A features to show it on screen, unaltered, using a combination of the techniques available to us. Any chance you can give me a sample of some 16 color art you'd like to get running on the system? I'd be happy to give it a go!