So I've finally been playing this game again (first time Final Mix) and this struck me. What if Donald had just said "Sure mate, welcome aboard!" when Sora invited Riku to join in Traverse Town? There seems to be room for 4 in the gummi ship, Riku just proved his ability to dispatch Heartless with ease and he even held the freakin' Keyblade without it teleporting back to Sora. What more proof that this kid just might be incredibly useful do you need? Maybe Riku left before hearing his answer, or maybe he would have refused because he couldn't stand playing second banana to Sora, but part of me likes to think that Riku owes his entire story arc to Donald Duck. At least he gave him and Maleficent something to bond over; the latter knows all about not being invited.