Mega Rayquaza confirmed, folks! Its new ability is Delta Stream, which cancels out all weather-based abilities and attacks, including Desolate Land and Primordial Sea while Mega Rayquaza is in battle. It also makes all Flying weaknesses neutral (Rock, Ice, Electric), making Mega Rayquaza neutral to Rock, 2x weak to Ice and resistant to Electric. It also gains a new signature move, Dragon Ascent, a supposedly high-BP Flying move. Rayquaza finally has a Flying STAB to abuse outside of lolFly and Air Slash.
I don't know if Mega Rayquaza will still have to use V-Create now that it has a nuke in Dragon Ascent. Anyway, possible sets:
-Draco Meteor
-Dragon Ascent
-V-Create/Overheat/Earthquake
-Extreme Speed
Nature: Hasty/Naive
EVs: 4 Atk/252 SAtk/252 Spe or 252 Atk/4 SAtk/252 Spe
Item: Rayquazanite
or (assuming Dragon Ascent is a physical move, which it looks like it is)
-Swords Dance
-Dragon Ascent
-V-Create/Overheat/Earthquake
-Extreme Speed
Nature: Jolly/Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Rayquazanite
or
-Dragon Dance
-Outrage
-Dragon Ascent
-V-Create/Extreme Speed/Earthquake
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Rayquazanite