I think it's best that we try to come up with many possible reasons ignoring how 'natural' or 'stupid' it sounds. This is what I usually do when brain storming. Bring out as many possibilities as possible without thinking about how reasonable it sounds. The more numbers the better. Quantity>Quality.
THEN we can look into each one of them and erase the most unlikely ones.
Here I go:
Why Himawari has whiskers
-It is a genetic trait and Himawari got them from Naruto.
-It is a genetic trait and Hinata got them from Toneri. (Maybe Toneri has whiskers hidden in his hair).
-Himawari is a daughter from Hinata's IT. Hinata's inner desire to have a child who resembles Naruto is what created the whiskers and the blue eyes(instead of Byakugan which is an aggressive trait.)
-Like Rin, Hinata had a bijuu/some other similar creature forcefully sealed inside her.
-Like the KinGin brothers, Hinata was trapped inside a bijuu/Toneri and digested their chakra that gained her the whiskers.
-Like Uzumaki Mito, Hinata sealed a bijuu inside her.
I repeat. The above options aren't supposed to make sense. The purpose is to get out as many ideas out as possible. Oh and btw, thinking of Kishi, the idea that is most unlikely might be the most likely outcome.
Edited by ramenanmitsu, 15 November 2014 - 12:53 PM.