Well, I examined the image closely, and it's not quite doable, but not far from it.
It's nicely put down to the 16 colors of the TI video chip, but to be perfectly doable (including the parallax scrolling), the video chip would need to support 2 planes of bitmap graphics having 4 colors each, and those 4 colors would have to be allowed to be freely used on pixel level. Both of these details are not fulfilled by the TMS9918A. In addition to that, it would have to allow sprites which use all 16 colors at once instead of only one.
But while it's not perfectly doable, it's actually close to being doable. I've run the picture through my converting program, and this is what it came up with:
As a proof that this picture is possible, I've also attached the data file my converting program produces:
This file contains the pattern and color table needed for the TMS9918's bitmap mode. So you first would have to put it into bitmap mode, and then copy the data into the video RAM at the position pointed to by the registers.
However, I'm not sure if the correct pallette has been used here... I did my best to map the colors used in this picture to the colors available on the real thing, but there seem to be three shades of cyan while the VDP only supports one. According to their use, I've mapped the darkest shade of cyan to dark green, and the medium one to light blue, so the picture may be somewhat off if displayed on a real TMS9918.
Also, there probably would be some issues with the animation. First of all, the main character, in this picture, has been rendered as being a part of the bitmap picture. Obviously, this probably wouldn't work in a game, so it and also the enemies probably would have to be converted to sprites, or use full characters which doesn't look too good in terms of separating the moving characters from the background. Also, the picture obviously is of one part, so parallax scrolling wouldn't work at all. There even is the question how you actually store all the graphics data needed and put it into the VDP reasonably fast to provide some sort of scrolling. Smooth scrolling probably won't be possible at this level of graphics detail, but scrolling in character steps (like in Slymoids) might work if you manage to compress the graphics data suitably. I think you would have to make a compromise between richness, colorfulness, complexity and speed of the background graphics. After all, you only have 48K of RAM (including the VDP RAM). Well, maybe on a big bankswitched cartridge...
Oh, just for the record, there has been a version done for the ZX-Spectrum which is basically more limited in graphics terms than the TI-99 is. That version has been converted to using just two colors for all of the game action, so maybe it might be possible on the TI-99 this way or similar to it:http://www.mobygames...ast/screenshots
Surely some form of Shadow could exist on the TI. Think Slymoids + Moon Patrol