Swampert-M Stats: 100/150/110/95/110/70 Type: Water/Ground - Physical attacker with moderate bulk. Kind of slow though, so I'm not sure he'll replace Mega-Gyrados except when Gyrados' weaknesses are a liability. Can go no weaknesses with Sap Sipper though.
Sceptile-M Stats: 70/110/75/145/85/145 Type: Grass/Dragon - Fastest grass-type by 18 points, fastest Dragon by 22. Also, the hardest hitting Grass-type, specially and competes for Grass physical nicely, mainly because of the power + speed combo. I suspect it'll outclass Skymin (which is yay and nay since I like both Pokemon) except for Skymin's Serene Grace sets. Could go Contrary with Leaf/Meteor, support with the fastest Spore/Seeds in the game that's also naturally immune to them, or lots of other stuff. A little frail though, and it doesn't outspeed Aerodactyl-M, who has an advantage over it.
Sableye-M Stats: 50/85/125/85/115/20 Type: Dark/Ghost - Slow as heck, but could use Prankster. Decent, but not amazing bulk. Still, only weakness if Fairy, so it could easily have a niche. Probably won't be common on most teams though.
Altaria-M Stats: 75/110/110/110/105/80 Type: Dragon/Fairy - Unique typing and usable stats, but it's a little on the slow side and doesn't seem to excel at anything except typing. It'll probably have some sort of niche by typing alone, but I don't expect common use.
Gallade-M Stats: 68/165/95/65/115/110 Type: Psychic/Fighting - Just straight up outclassed by Xtwo.
Audino-M Stats: 103/60/126/80/126/50 - Speed liability, but usable bulk and its Fairy-typing cancels its Fighting weakness, which is nifty. Pretty much a Fairy with a Spooky Judgement immunity, something Gengar might not be too fond of. In theory, could make for a good check/counter to offensive Giratina since it's immune to both of its STABs.
Sharpedo-M Stats: 70/140/70/110/65/105 - Despite the speed advantage, it still hits softer and is frailer than Gyarados. And it only notably outspeeds base 100s, like Regigas. I think it'll be outclassed.
Slowbro-M Stats: 95/75/180/130/80/30 Type: Water/Psychic - That epic defense stat without being a Steel or Rock type. A bit frail specially, but it could easily pair up with a partner to handle that. Also has a usable Sp. A to hit stuff back with. It'll probably make for a great physical Contrary set since it's going to resist to hell out of V-Create and Super Power.
Steelix-M Stats: 75/125/230/55/95/30 Type: Steel/Ground - Pretty much another flavor of Aggron. Picks up some weaknesses, but Ground does give it an immunity to Electric, so it'll have a use somewhere.
Pidgeot-M Stats: 83/80/80/135/80/121 Type: Normal/Flying - Pretty decent power and speed, which makes it faster than other Flying special attackers. But, that'll probably be it's only niche.
Glalie-M Stats: 80/120/80/120/80/100 Type: Ice/Ice - Bad defensive typing plus not great bulk plus not really excelling anywhere kind of makes this one a bit dissapointing. It outspeeds the Kyu-Bs, but that's about it. Weaville would probably still be a better physical attacker simply because it's faster.
Diancie-M Stats: 50/160/110/160/110/110 Type: Rock/Fairy - Xern now has some competition, although Diancie suffers defensively because of that Rock-typing. Water weakness, Grass weakness, Fighting neutrality, 4x Steel Weakness... yeah... I think Xern is also bulkier because of it's HP stat and only having uncommon weaknesses, so it'll probably work better for most defensive roles.
Metagross-M Stats: 80/145/150/105/110/110 Type: Steel/Psychic - Looks like a reasonably fast, bulky physical attacker without shabby special defenses. Metagross has always been just a little weaker than desirable and this Mega gives it just what it needed. At the very least, it outclasses Jirachi now.
Kyogre-P Stats: 100/150/90/180/160/90 Type: Water/Water - Dear god, this thing is a monster. Drizzlespout, Regen Vest, Prankster, PH Q. Dancer, this guy is going to be crazy. Hell, it can even run physical attacking sets with that base 150 attack. But on the bright side at least, Bolt Strikes will still end it very quickly.
Groudon-P Stats: 100/180/160/150/90/90 Type: Ground/Fire - Groudon is finally back after taking a hiatus during X/Y. This thing will be friggin scary as well. Just imagine it's Drought V-Create >.> And it also gets STAB on Thousand Arrows to piss off your Flashfire Steel types (go go Scizor!) On the other hand, Water will murder this thing if it's not running the right ability and Earth Power is going to scare the hell out of it regardless.
Rayquaza-M Stats: 105/180/100/180/100/115 Type: Dragon/Flying - Going to be the Aerilater from hell. And even if those abilities get addressed one way or another, this thing is still going to be one freaking scary mixed sweeper. All I can say is, prepare your Steel types. (Or Diancies). On the bright side, 4x Ice Weakness, so if Kyu-B gets in, Ray runs in terror. Or, if -ates get hit, then Ice Shards or really any other stray Ice attacks.
Hoopa-M Stats: 80/160/60/170/130/80 Type: Psychic/Dark - First thing that comes to mind is total U-Turn bait. Still though, those are some very nasty mixed offenses and not too shabby special bulk. A psychic that's not scared of Gengar or Yveltal will definitely be appreciated. On the other hand, whack it with a physical attack and Hoopa is done.
Camerupt-M Stats: 70/120/100/145/105/20 Type: Fire/Ground - Now if only Groudon-P wasn't a thing. But, at least Camerupt would be very good for Trick Room shenanigans.
Lopunny-M Stats: 65/136/94/54/96/135 Type: Normal/Fighting - I'm not sure if this would be able to compete with Meloetta-P or not. In either case, Xtwo is currently kind of kicking all of our other Fighting types to the curb, which kind of sucks.
Salamence-M Stats: 95/145/130/120/90/120 Type: Dragon/Flying - For when you absolutely, positively need that speed boost over Rayquaza. Otherwise, outclassed in every way except maybe physical bulk, and even then not by much.
Beedrill-M Stats: 65/150/40/15/80/145 Type: Bug/Poison - If BH had a glass cannon, this is it. I think this will be one of the few Pokemon who'd appreciate a Choice Item or Focus Sash more than anything else in BH since it doesn't have the bulk to stay in on just about anything. Its STABs are both underutilized, but they would threaten the hell out of stuff like Fairies and Hoopa-M. I guess this is what Deo-A would be like if it wasn't outclassed by normal Mewtwo.
Off-hand, if you don't mind stacking weaknesses, Kyo-P and Slowbro-M look like they'd make for a great defensive core. Funny enough, Kyo-P and Groudon-P also look like they'd do the same.