I decided to lead with Xerneas for a similar reason to why I led with Clefable last season. Xerneas can lead and use Geomancy to increase its stats, and if there's an opening I can either swap into another Pokémon that's better on field or attack with a Xerneas with boosted stats that would last longer than usual. I gave Xerneas the same Quick Claw I gave the Clefable, and it worked phenominally for Xerneas. That extra time gained on a Quick Clawed Geomancy made such a bigger difference than setting a shield on Clefable did, and this allowed Xerneas to snipe out more threats on board for longer before bringing in a soon-to-Mega-Evolve Gyarados to flood whatever is left on the field.
Mega Gyarados filled the exact same role it did last season. Part-time Dragon-type that can hit large areas of the board at once with less of the impact compared to the Full-time Dragon-types in the format. Base Gyarados can fill that gap until I need the Mega Evolution.
Finally, Glaceon filled the exact same role to a T. It existed to snipe annoying Dragon-types on the field, it came out to snipe annoying Dragon-types on the field. Its Occa Berry allowed it to survive long enough to take out more threats this time, or survive an initial blast so I can rush to swap in a Xerneas to take the second hit for it.