Nice find! I love all those sketches. I guess this finally confirms the (very likely) theory that Y Signal = Yellow Signal.
The downloadable documents show scrapped things like a car running into a wall in stage 1, as well as plans for a bike stage, much like the scrapped (and restored) bike stage of SoR 3. And what looks like a unique Blanka-like boss for stage 6, as well as some fencer guy...
Anyway, I'm working on a beat 'em up myself. I'm glad we don't have such limitations anymore (tiles, memory, colors...), or it'd be way too much work.