This is sweet stuff but the scale looks quite off for OPP:

I think he may have a hard time walking through that door.

There are no standards unless you are developing for specific hardware (really only if were talking very old hardware) or engine.

If you want it to be widely useful that the best bet is to keep it as simple and easy to use as feasible.

For example a platformer could have one in front of sprites foreground layer, one behind sprites forground layer plus an optional parallax background layer which should be sufficient for most cases. This would allow for grass drawn infront of a character's feet and a ladder drawn behind the character.

You reckon sprites to fit 64*64 would be a suitable pixel scale?

All vehicles traction driven for consistent physics, or hovery stuff too?

EDIT:

How about something along these lines:

Seperate sprites for weapons drawn on top of vehicle. EGA global palette.