This is massive new or interesting info gathered from the first trailer of Pokémon Legends Z-A.

The focus is entirely on Lumiose city and one other location through various time periods.
Z stands for Zygarde
A Represents Celebi and its ability to revisit the past.

The starters will be: Snivy, Litten, and Piplup.
(No words on Mega evolution or regional forms)

They show Mega Flygon, Mega Noivern, and Mega Dragapult.

Battles will have phase shifts - a new mechanic that requires live-time inputs and control of the character and Pokémon.

A new pokemon evolving from Malamar is shown but not introduced. Shown in the phase shift during a montage.

There are cries of Xerneas and Yveltal throughout the entire trailer.

A Zygarde core seems to sit on your shoulder or hat, at one point the 10% form is seen running alongside the player.