In a recent interview where the directors were re-asked about some theory they gave during a Reddit AMA, Chris Buck said the following.
"‘Of course Anna and Elsa’s parents didn’t die. Yes, there was a shipwreck, but they were at sea a little bit longer than we think they were because the mother was pregnant, and she gave birth on the boat, to a little boy. They get shipwrecked, and somehow they really washed way far away from the Scandinavian waters, and they end up in the jungle. They end up building a tree house and a leopard kills them, so their baby boy is raised by gorillas. So in my little head, Anna and Elsa’s brother is Tarzan.”
The director went on to say that because all of this is make believe that he can go crazy. "On the other side of that island are surfing penguins, to tie in a non-Disney movie, Surf’s Up. That’s my fun little world.”
Tarzan probably isn't showing up in the Frozen sequel.
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what Disney animated properties would you like to see crossover?