@SLotman: People told me that my little voxel scene looked like your idea, so I'm dropping this here :-) It's not really comparable though as this is definitely NOT something an emulator would ever be able to produce. The only way to do something like this would be to do a full remake, but I'm not up to that task. Way too much work ;-)
Anyway, really liking your gif. Not sure if it would be fun to play, but it sure looks amazing!
Nice; I really like your voxel mockup; image saved 
...is there more where that came from?
Actually, doing that in emulators could be possible... The main problem I see is again that sprites are rendered line by line - if they were rendered as single shapes, you could just use some sort of "color/height" thing to do it.
You could render each pixel as a small cube, the Columbia sprite for example, white being "taller" and blue being half height - wouldn't be perfect, but it would give some cool results 
Just a small test - grabbed the sprite, and rendered it like I said, as a small bunch of cubes, with different heights for each color... this is the result:
Cool idea, and I think it might actually work. I just very quickly hacked together this idea in an MSX1 emulator I wrote myself. Right now it only works with Knightmare. See screenshot.
I simply applied a transformation to turn the rectangular drawing area into a trapezoid. The sprites, as you can see, are handled differently. I simply draw the sprite at the desired (transformed) location, but without applying the transformation on the sprite itself. One thing that became obvious is that different sprites need to be handled differently (e.g. knight should be drawn upright, but arrows shouldn't) and figures consisting of multiple sprites (such as the knight) need special handling, to avoid different parts being drawn at a different "depths". You can see this in the screenshot, where the black and white parts of the cannon balls do not coincide.
Obviously my implementation is still quite ugly and needs more work. If anyone wants to pursue this idea seriously, one thing to be done is to parametrize the way all (0...31) sprites are handled, and to create profiles for different games. As I said, I only looked at Knightmare (sprites 0...3 are the knight, 4...5 the arrows, etc.). Other games will have other sprites assigned to these numbers.