I doubt Game Freak wil go this route, but I hope that Mega Sceptile gets an Attack increase and Mega Swampert a Special Attack increase. Resasons for this is that Sceptile can boost its Attack with Swords Dance, have an Adamant nature and still outspeed Greninja, the fastest OU relevant attacker after Deoxys-S, as long as its new Speed stat is 140 or higher. Dragon STAB provides good neutral coverage while his other moves cover some weaknesses (Leaf Blade knocks out Hippowdon, Quagsire and Azumarill; Earthquake kills Mega Mawile and other non levitating Steels and Rock Slide knocks out Talonflame and Mega Pinsir on the switch). Unlike most sweepers, Thundurus can't paralyze him with Thunder Wave.
As for Mega Swampert, his Special moves have higher base Power than his Phyisical moves. Also, Mega Swampert doesn't have the option to boosts neither his Attack (Kabutops has Swords Dance) or his Special Attack (Kingdra and Ludicolo at least can hold items) and he can't hold a Life Orb. Although if Mega Swampert doesn't keep up with the rest of Rain Dance sweepers he can attack with both ends of the spectrum as a standalone rain sweeper.
As for Mega Swampert, his Special moves have higher base Power than his Phyisical moves. Also, Mega Swampert doesn't have the option to boosts neither his Attack (Kabutops has Swords Dance) or his Special Attack (Kingdra and Ludicolo at least can hold items) and he can't hold a Life Orb. Although if Mega Swampert doesn't keep up with the rest of Rain Dance sweepers he can attack with both ends of the spectrum as a standalone rain sweeper.