Actually, none of the trio were supposed to happen! It is after all canon that Sora is a walking WTF right next to Kairi and Riku. That is what makes them interesting and also delightful. After all, Terra, Ven, and Aqua were supposed to happen, as were Eraqus and Xehanort, and look what happened to them. And I'm pretty sure that Lea/Axel wasn't a thing that was supposed to happen either. I'm not counting Mickey because there have been hints here and there that Mickey might not have been a thing that was supposed to happen either. (Why are you so comfortable in the darkness, your majesty?)