Here is your leak. Main Hoenn trio gets "past forms". They are called Prime forms in japan
They are all small changes.
Kyogre just gets more longer snout, I guess they wanted to make it more like megalodon. Rest of the body dont change
Raquaza becomes longer and has arceus cross on the middle
Groudon becomes more devilish, eyes have frienzied look.
They all have new abilities that summon weather but with interesting twist.
Groudon summons "Blazing star" - That damages all types but fire, ground and rock also summoning sun
Kyogre summons "Glacial clouds" - That damages all types but ice and water also summoning rain
Rayquaza summons "Wind Force" - That damages Every type but Dragon and Flying also giving speed boost