ITT, we come up with potential movesets without even knowing final stats for the new megas.

>Mega Altaria
~Rest / Roost
~Sleep Talk / Cotton Guard
~Hyper Voice / Moonblast / Body Slam*
~Flamethrower / Earthquake
*Body Slam being a Gen 3 Tutor.

>Mega Lopunny
~High Jump Kick / Drain Punch*
~Retaliate / Fake Out / Facade
~Fire Punch
~Ice Punch

>Mega Salamence
~Thrash
~Earthquake
~Dragon Claw / Fire Blast
~Dragon Dance

>Mega Sableye
~Shadow Ball
~Will o Wisp
~Recover
~Foul Play / Knock Off / Sucker Punch