In Kingdom Hearts 3 Mickey will give up his body in a manner similar to how xehanort did so and travel back to when he failed to save his "3 friends". I was watching dream drop distance cutscenes and he was extremely sorrowful that he was unable to save them when he confronts xehanort about it. Although "destiny" cannot be altered in the restrictions of the time travel bull shit I do not believe for 1 second that Ventus, Terra, and Aqua's "destiny" was to be in the positions they are now. There are many reasons supporting this but you must also refer to the doubt in this theory. Ventus will likely awaken soon and Aqua will maybe be saved. As for terra he will be a dark vessel so he's out of the picture for the time being. I believe if Mickey were to travel back to the confrontation with ven, terra and Aqua he would be able to prevent terra becoming xehanort, ven going into a slumber and Aqua from falling to darkness. This is just a theory please have an open mind when responding and please do not speak for square and act like it automatically won't happen or something else will happen

Number one you can't go back and change the past with out breaking the laws of time as stated by Young Xehanort. Saving Terra, Aqua , and Ventus in the past would severely alter the timeline. The events of BBS were what set the forth the events of the remainder series as elaborated by Master Xehanort ("The broken boy who failed to be the blade. The misguided master who sacrificed herself for a friend. And the feckless youth who became my new vessel.") Terra, Aqua, and Ven's sacrifices are what bridged the events of the past with the present stream of events.

Yep, basically this.