Mega Braviary
Typing: Normal/Flying ----> Normal/Flying
Abilities: Keen Eye/Sheer Force/Defiant ----> Sheer Force
BST: 100/123/75/57/75/80 ----> 100/148/110/57/95/100
New Moves: Rock Climb, Flame Wheel, Crunch
For some reason there are only two megas that uses Sheer Force, an ability which I think is on par with the likes of Tough Claws and Technician. Braviary originally is quite lacking in moves that are boosted by Sheer Force, with basically only Rock Slide and Steel Wing for its use. However, now with the addition of Rock Climb, Flame Wheel and Crunch into the movepool, it has much more moves that can be used. 100/110/95 bulk is pretty good, and gives it many occasion to setup Bulk Ups. It can also nab the Defiant boost before Mega Evolving, making it even more powerful.
Mega Mandibuzz
Typing: Dark/Flying ----> Dark/Flying
Abilities: Big Pecks/Overcoat/Weak Armor ----> Bone Spike (Clone of Iron Barbs/Rough Skin)
BST: 110/65/105/55/95/80 ----> 110/115/140/55/120/70
New Moves: Drill Peck, Poison Jab, Bulk Up
Mega Mandibuzz becomes even more of a tank, being even bulkier than Skarmory on the physical side as well as Heatran on the special side. It even have Bulk Up to further buff its Def! Bone Spike can slowly whittle down physical attackers while Mega Mandibuzz setups, and it now has Drill Peck for a non-recoil Flying STAB, as well as Poison Jab against Fairy types. However, due to its otherwise lacking movepool, it's completely walled by the likes of Mega Mawile and Bisharp. Seriously, its only coverage against them is Bone Rush.
Mega Heatmor
Typing: Fire ----> Fire/Water
Abilities: Gluttony/Flash Fire/White Smoke ----> Storm Drain
BST: 85/97/66/105/66/65 ----> 85/132/81/140/66/80
New Moves: Scald, Hydro Pump, Aqua Tail
Basically just buffed offense all around, and an extra Water secondary typing. Fire and Water type compliment each other rather nicely, and Storm Drain further improves its already decent Sp.Atk. It can also use Hone Claws to boost up its Atk with the help of its slightly buffed Def. However, Mega Heatmor is still frail on the special side, and it isn't exactly that fast as well, sitting at the 80 Speed mark.
Mega Cobalion
Typing: Steel/Fighting ----> Steel/Fighting
Abilities: Justified ----> Overwhelm (Uses Taunt on Mega Evolution and switch-in.)
BST: 91/90/129/90/72/108 ----> 91/135/139/100/97/118
New Moves: Extreme Speed, Thunderbolt
I'm going to give each member of the muskedeer quartet different abilities, so they can have different roles. Mega Cobalion here is specifically designed against Stealth Rock setters. It can hit most Stealth Rock setters for SE damage, and even have Thunderbolt for Skarmory. It also have a decent speed, bulk and attack, allowing it to be a viable set-up sweeper with Swords Dance. Overwhelm shuts down many supporters, often causing switches which gives it space to setup. Extreme Speed is also added to its movepool if it wants to do revenge kill.