Honestly, let's say Pan's Labyrinth (which was Rated R here) should be a Rated 15. It has some gross moments, but in the end, they're harmless. Not to the point where it can cause damage to the public.
Yeah, that should really have been a PG-13. Not a lot of language and blood from what I remember. Although it did have a shot of a flying monster...that didn't have a shirt on.
This is what we have...
G
PG
PG-13
R
NC-17
This is what we need...
G
PG
PG-13
R-15
R
NC-17
As you can see, R-15 would be the new and lone new rating. It would actually be juts a light or maybe even medium R. Look at The Punisher (Thomas Jane version) I thought it was a light R. Saw is another example of Light R. Not a lot of blood...but it does have tension. Personally, I think a 15 year old can handle Saw. Saw III would be NC-17, but MPPA thought otherwise and gaved it an R.
Watch again, it showed a few bare breasts in the movie when Sheila first meets Evil Ash the deadites are rounding up women & pulling them around with chains you can clearly see that they aren't wearing any top.