>Story
Is actually in the future, set 60 years after XY, focuses on rebuilding areas of Lumiose after the Ultimate Weapon was rebuilt by a new evil group called Team Light and fired upon Lumiose.
A man that looks like Briar is the leader. This is the first time they really touch death so heavily since Lavender Town or Mt. Pyre.

>Pokémon
Starters are Snivy, Scorbunny, and Totodile, and each get regional forms. Not sure about the types but Cinderace looks like a Fire/Fighting Luchador.

Lapras evolves into called Laprace that looks like a speedboat and is used for racing on the canal. It can Mega Evolve and stays Water/Ice.

Gogoat evolves into Gogruff and just gets slightly bigger horns and rocks along its body, Grass/Rock probably.

Psyduck has a stressed out looking regional form, seemingly stressed from the constant noise of the city.

Last I know of is a convergent species that looks like Beldum, but like a pitcher plant, seen outside florists stores.

>New Megas
Venusaur-X and Blastoise-X, Chimecho, Rhyperior, Raichu, and the Kalos starters are some of them, not sure how many there are in total.

>Rideable Pokémon
Like in LA, there's mounts for each terrain. Gogruff is used for land travel, Ariados is used for scaling walls and Laprace for swimming are all I know of.

>Gameplay
Abilities and held items return, and all the old mechanics from LA are returning.
After the Ultimate Weapon was fired, Lumiose began to overflow with Tera energy and Pokémon started rampaging, like the nobles in LA.
Tera Raids are also returning and receive a massive overhaul, being less buggy and more responsive.

As for online play, you can trade AND battle, and something similar to the lost bags from LA return.