My source recently sent me this list of unrevealed new Mega Pokemon for Omega Ruby and Alpha Sapphire.

Mega Gallade - Psychic/Fighting - Sharp Blade (boosts the power of slashing moves by 50%)
Mega Sharpedo - Water/Dark - Speed Boost
Mega Camerupt - Fire/Ground - Water Absorb
Mega Torkoal - Fire/Steel - Filter
Mega Flygon - Ground/Dragon - Arena Trap
Mega Zangoose - Normal/Fighting - Guts
Mega Seviper - Poison/Dark - Infiltrator
Mega Milotic - Water/Fairy - Marvel Scale
Mega Metagross - Steel/Psychic - Iron Fist

Received from Professor Birch post-game:

Mega Meganium - Grass/Fairy - Natural Cure
Mega Typhlosion - Fire/Ground - Flash Fire
Mega Feraligatr - Water/Dark - Strong Jaw