I'm not a leaker so I'm going to spoiler you some parts of the game;
The game is complete, demo going to be released in next month

Starters;
Owl evolves into archer, the middle stage is based on some noble [this line es based on a nobles who decides to become bandits- robin hood variation]

Tiger evolves into wrestler, the middle stage is based on teen kid who decides to enter the street gang [this one is based on a story of poor kid who became pro wrestler]

seal evolves into siren, the middle stage is based on a some middle class/normal person [this one is based on our talents to becoming whatever we want and also reference to the ugly duckling story]

We going to return to Kalos in february DLC where we going to meet Blue who is spying team flare and kalos e4 member

Red is training inside of volcano but we don't have access to him until post game

No league, we going be able to chose new league EVERY TIME picking from trainers we met. They going to fight each other and decides champion among them first in battle royale then in 2v2 [everything happens randomly] the game prepared different teams for each

Evil team are scientist's

Corless appears in kalos

Zygarde story have connection to the kalos event's and basically sun&moon is complete kalos storyline, Lysandre escaped and is going to take control on both xerneas and yveltal

AZ going to die from Lysandre machine and his special pokemon is going to roaming around the world [he released it before he died]

Mega evolutions;
Butterfee
Shuckle
Johto starters
Kalos starters
Mew
Machamp
Zebstrika
Golduck
Poliwrath
Politoed
Crobat
Porygon Z
Flygon
Huntail
Gorebyss
Wailord
[bunch of hoenn mons]
Raticate
Nidoking
Nidoqueen
Legendary birds
NO 4 gen mons [porygon is gen 1]
Dugtrio
[bunch of kanto mons]

this gen really decides to fill lacked megas

there's battle frontier in kalos DLC in lumiose city