I wouldn't put it past Yoshi P to order it to be a bit different for a crossover this time. It could be more extensive than past crossovers, because of his involvement in the game. It is his baby, and he will likely want to make the crossover something extremely special, and dedicated to it. I wouldn't be surprised if it were a trial in all honesty, or if they went to a great length to include a lot of mounts, minions, weapons, and glamours.
Another possibility is it could be added in more than just a single crossover. They could add new veteran rewards, gold saucer, and other methods of gaining these items too. It wouldn't entirely surprise me if for the next fan fest we got XVI minions as the bonuses. Though, for those they usually go with minions that match the inspiration for the upcoming expansion, they could always break tradition in favor of XVI.
Though XVI being the inspiration for an expansion isn't out of the realm of possibility. Before XV came out we got a lot of Thordan, and Knights of the Round related stuff which was tied to XV in a big way. Especially with the movie Kingsglaive. Its clear that Square Enix was priming our familiarity around these classic summons. I noticed that before XVI, we gradually got a lot more phoenix content. A fire bird inspired glamour on the mog station, TWO extra phoenix mounts, etc. Much more than just the stuff we got from Suzaku. It feels intentional.
I also noticed around the time of the FFVII: Rebirth release we gradually got new FFVII stuff in the form of the weapons. Square Enix may be influencing the devs of XIV in ways than we normally realize. It wouldn't surprise me if board members approached Yoshi P, and were like "This game brings a lot of money and attention. Can you think of a way to incorporate things that have to do with ____?" It doesn't sound unlike Square Enix at all.