I think a great addition to the game would be a few particles in the decoration tab that allow other particles to either pass infront of it or behind it. I'm not too good at game making, so I dont really know how to make a demo, but it would kind of just be an interchangable background.
GRVT sort of does that if you set its life, vx, vy and tmp to 0; and I've been using it to hide sparks for quite a long time. You can layer it over (or under) anything by careful particle order manipulation (because whether it's drawn before or after the other particles in the stack depends on its ID).
I actually have a script that adds a DGRV element to the same section GRVT is in, but the moment you place it down it becomes GRVT with those properties set (I call it deco GRVT, hence DGRV). I can publish it on the Script Manager if it turns out that it's actually useful.
What I would love to see is conductivity in layers. SPRK won't go through any conductor that is covered by another element. It doesn't make sense, since fire burns when layered over, and some other things are not affected either.
It would be so cool to be able to run a wire through a brick wall in a truly realistic way. [sigh]