Now the first question is—which elf? Santa's elves? Shoemaking elves? Keebler's cookie-making elves? Tolkien's elves? The Dökkálfar (dark elves) and Ljósálfar (light elves) of the original Norse mythology?
My first proposal is this—let's mix the latter two together, the "dark" and "light" aspects reflecting an African-European division. The reason I'm mixing Tolkien's elves with the original elves is that both categories are quite prominent in the high fantasy subgenre.
Now the most obvious difference between us and elves is the pointy ear. In the mammal world, this isn't so far-fetched.
Foxes have pointy ears.
Bats have pointy ears.
The fennec fox's ears are just too big for someone supposed to reside in the frigid forests of northern Europe, and a bat's ears are too naked for the same thing. So my proposal is this—don't make the ears themselves pointy. Instead, let's keep the human ears but cover them with hair, going as far as giving them tufts, just like the ears of a lynx.
Another obvious difference between us and them is that elves are immortal. Now, biologically, we can't make any lifeform truly immortal, but we CAN lengthen the telomeres, repetitive nucleotide sequences at the base of each chromosome, to make them live longer. A "biologically immortal" organism, as scientists would call it, would still die, but senescence (think "senile") would not be an existant cause. My proposal is that we lengthen the telomeres to the extent that the average elvish lifespan is triple-and-a-half greater than the average human lifespan.
This would suggest altering the elvish metabolism from endothermy (generating an internal body heat system, therefore keeping temperature constant) to mesothermy.
As always, enlarged respiration and tetrachromacy will include the elves on the list of candidates.
Tolkien described his elves as "slender" and "graceful, yet strong". My proposal to make that description more concrete is to, essentially fit a musculature as dense as a Bowflex body onto a skeleton as slender as a gibbon's (while at the same time retaining the human proportion.)
My final proposal differentiating elves from humans may be the most radical — hermaphroditism. To be specific, simultaneous hermaphroditism, which means that each and every adult has both male and female sex organs simultaneously active. Unlike humans, there is no clear-cut distinction between a male elf and a female elf.
Are any of my proposals listed above sound, or have I created some unintentional side effects to the elvish body?