As much as I hate to announce this, Mega Slowbro and Mega Audino are confirmed.
Mega Slowbro retains its Water/Psychic type and has Shell Armor. Mega Audino is now Normal/Fairy with Healer (one of vanilla Audino's regular abilities, in fact).
I think they're rather underwhelming, IMO. The loss of Regenerator for Mega Slowbro kinda hurts. However, Calm Mind Mega Slowbro at least has some use, as Shell Armor blocks critical hit hax from ruining it. Slowbro can also hold off of Mega Evolving on purpose to take advantage of Regenerator early game, then Mega Evolve late-game.
Mega Audino, on the other hand, appears to be geared towards doubles; Healer is useless in singles. Of course, Mega Audino faces stiff competition from Aromatisse as a doubles supporter and other Megas for the Mega slot.
Anyway, potential sets:
Mega Slowbro:
-Calm Mind
-Scald
-Psyshock/Ice Beam/Fire Blast
-Slack Off
Nature: Bold
EVs: 252 HP/232 Def/24 SDef
Item: Slowbronite (assuming that's the name of the Mega Stone)
Ability: Regenerator
Mega Audino (doubles only):
-Trick Room
-Heal Pulse/Safeguard
-Dazzling Gleam/Hyper Voice/Return
-Protect
Nature: Sassy/Relaxed
EVs: 252 HP/4 Def/252 SDef or 252 HP/252 Def/4 SDef
Item: Audinonite (assuming name of Mega Stone)