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If the creators went with old imperial traditions then the next heir would be Neji, since he is the son of the head of the clans, dead brother. If all of the males in the clan were dead then Hinata would have to marry so that her husband would become the next head. Of course, there are ruling females, but they are believed to be temporary until another male heir is found, which can take years.
Though this is the Naruto world, and traditions and law are similar but not the same, so we can only hope and assume who will be heir of the Hyuuga clan.
