Why did Kishi draw the One Piece symbol in the last page of the last chapter of Naruto? Why did he put a One Piece skull and crossbones (along with the straw hat) on the very last page of the last chapter of Naruto? Seems odd, as the authors differ in both manga (Kishi - Naruto, Oda - One Piece), and I don't understand why.
Oda also made a reference to Naruto in the latest chapter (766):

Notice Nami's dress with Konoha's symbol on it, and how there's a fox eating ramen with a whirlpool on its back.

It's just a way for them to acknowledge each other's work.