I've always been of the opinion that "half elf" really includes others that are more than just straight human + straight elf, plus there will always be some degree of variation with appearance (someone that is 80% elf and 20% human looks really different than the other way around).

That being said, I would like to suggest some culture options for half elves:

vaguely
partially
somewhat
mostly
more or less
faintly

Example:
She appears to be vaguely Elven.
She appears to be mostly Human, and somewhat Elven.
She appears to be faintly Human, but mostly Sylvan.
She appears to be vaguely human, but mostly Elven or Sylvan.

I don't know how hard it would be to implement the variations, but I thought it would be fun.

~ The girl behind Debia

---

You swing an invar-hilted serrated glaes falchion at an Ithzir seer!
Amazingly, the Ithzir seer manages to parry the attack with her staff!
The scintillating Debia swings an invar-hilted serrated glaes falchion at you

My thoughts were always:

Human/Elf = Pure-blooded human/elf, or close enough to pure-blooded that you pass.
Half-elf = Obviously mixed-blood.

While it'd be nice, I'm paranoid about dev time and not sure a new system is necessary; this kind of thing is handled nicely already with feature alterations. I just wish feature alterations (and alterations in general) were more readily available.