Of course, there is a lot of speculation and theories going around that Kuroyukihime from accel world is Kirito and Asuna's daughter (second eldest I would assume cuz they have Yui :P). I believe this/want it to be true because... Well... It'd be awesome! (Kirito - the first one to clear SAO. Kuroyukihime - the first to clear accel world. Like father like daughter? Lol)
Anyways, here's some evidence that I've gathered up without actually looking at other people's theories. Firstly, to answer the question "how would that even be possible?" SAO and AW are set in the same universe. In AW kuroyukihime is 14 in the year 2046, therefore the year she was born is 2032. As for SAO, Kirito is 16 in the year 2024, so in 2032 he would be 24 (Asuna, 25) which is not an unreasonable age to have a child.
Of course... There's also the thing that kuroyukihime has Asuna's eyes and Kirito's hair colour, that helps.
And as we know, one of the requirements for accel world is to have the neural linker since birth, and Kiriot seems like a parent who would really advocate for that, as we all know that he loves new dive technology :D
Therefore I think there's a high possibility that kuroyukihime is Kirito and Asuna's daughter. Yay!
Please comment with any of your own theories as I would like to hear what everyone else can come up with :)