What would your storyline for Final Fantasy XV, the milestone, be?
For me, it would be a hodge podge. Bosses and the main villains from every previous installment would be brought together by a mysterious force that is attempting to bring all the worlds before it together to cause chaos. Two or 3 characters from each previous game would be sent through a rift to stop this, along with 2 or 3 new playable characters, and give you a large # of playable characters. Cloud and Tifa for example. The world map would be an amalgamation of the world maps from every previous game, until the final area which is all new where you encounter the force that is trying to bring all this destruction about.
Granted, Square probably wont do this, but I think it would a great side project(like Kingdom Hearts).