I'm only assuming this, because she has purple eyes and the Eyes of the Falmer (Gems in Statue in Irkngthand) are purplish pink.
Rare coincidence or a hope for the Falmers to return to Skyrim?
The Falmer can't return when they've been there the entire time. And as has been covered by Cecliff, we already know who Karliah's parents were.
Even if she was the spawn of a Falmer, then it was probably a male Falmer's assault on a Dunmer woman, seeing as the Falmer haven't been themselves in thousands and thousands of years, and Karliah is certainly not that old. Either way, the old Snow Elves that weren't goblins are long gone. One child isn't going to change that.