Why are you investing in Hydreigon's Special Attack when it's clearly a special attacker? Choice Scarf or Life Orb should be the item.
Blaziken, Aegislash and Mega Mawile are Ubers, so they can't be used in OU teams. Gengar is a good Ghost substitute, while Mega Heracross, Mega Gallade, Mega Metagross and Mega Charizard X are substitutes for Mega wall breakers:
Gengar:
-Sludge Wave/Sludge Bomb
-Focus Blast
-Destiny Bond/Taunt/Substitute
Nature: Timid
EVs: 252 SAtk/4 SDef/252 Spe
Item: Life Orb/Focus Sash/Black Sludge
or
-Substitute
-Shadow Ball
-Will-o-Wisp
-Taunt/Sludge Bomb/Sludge Wave/Disable
Nature: Timid
EVs: 28 HP/148 SAtk/80 SDef/252 Spe
Item: Black Sludge
or
-Hex
-Taunt
-Will-o-Wisp
-Sludge Wave/Sludge Bomb/Substitute
Nature: Timid
EVs: 8 HP/248 SAtk/252 Spe
Item: Black Sludge
Heracross:
-Pin Missile/Substitute
-Close Combat
-Rock Blast
-Swords Dance/Substitute
Nature: Jolly
EVs: 4 HP/252 Atk/252 Spe
Item: Heracronite
Ability: Guts/Moxie
Gallade:
-Swords Dance
-Zen Headbutt
-Close Combat
-Knock Off/Ice Punch
Nature: Jolly
EVs: 252 Atk/4 Def/252 Spe
Item: Galladite
Metagross:
-Meteor Mash
-Zen Headbutt
-Earthquake/Hammer Arm
-Ice Punch/Bullet Punch/Grass Knot
Nature: Jolly/Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Metagrossite
Ability: Clear Body
or
-Agility
-Meteor Mash
-Zen Headbutt
-Earthquake/Hammer Arm
Nature: Adamant
EVs: 80 HP/252 Atk/176 Spe
Item: Metagrossite
Ability: Clear Body
Charizard:
-Dragon Dance
-Flare Blitz/Earthquake
-Dragon Claw
-Roost
Nature: Adamant/Jolly
EVs: 96 HP/252 Atk/160 Spe
Item: Charizardite X
Ability: Blaze
or
-Flare Blitz/Earthquake
-Dragon Claw/Earthquake
-Will-o-Wisp
-Roost
Nature: Careful
EVs: 248 HP/180 SDef/80 Spe
Item: Charizardite X
Ability: Blaze
or
-Flare Blitz
-Dragon Claw
-Earthquake/Substitute
-Roost
Nature: Adamant
EVs: 40 HP/252 Atk/216 Spe
Item: Charizardite X
Ability: Blaze
or
-Swords Dance
-Flare Blitz
-Outrage
-Tailwind
Nature: Adamant
EVs: 40 HP/252 Atk/216 Spe
Item: Charizardite X
Ability: Blaze
The only special moves worth running on Tyranitar are Fire Blast and Ice Beam; it's better off using Crunch over Dark Pulse, as it runs off of its better Attack:
-Stone Edge
-Crunch/Ice Punch
-Pursuit
-Earthquake/Superpower/Fire Blast
Nature: Jolly/Naive
EVs: 252 Atk/4 Def/252 Spe
Item: Choice Scarf
Ability: Sand Stream
or
-Stealth Rock
-Crunch/Pursuit
-Rock Slide/Ice Beam/Thunder Wave
-Fire Blast
Nature: Sassy
EVs: 248 HP/136 Def/124 SDef
Item: Leftovers/Smooth Rock
Ability: Sand Stream
Excadrill > Seismitoad. Seismitoad doesn't have much to make itself stand out in OU, apart from being a Swift Swimmer immune to Thunder Wave, but Mega Swampert overshadows it in that department now. Plus, Excadrill's Sand Rush pairs well with Sand Stream:
-Earthquake
-Iron Head
-Rock Slide
-Rapid Spin
Nature: Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Life Orb/Air Balloon
Ability: Sand Rush
As I mentioned before, Blaziken is an Uber, so Talonflame is a good substitute:
-Swords Dance
-Flare Blitz
-Brave Bird
-Roost
Nature: Jolly
EVs: 88 HP/252 Atk/168 Spe
Item: Leftovers/Sharp Beak/Sky Plate
or
-Flare Blitz
-Brave Bird
-U-turn
-Steel Wing/Roost/Tailwind
Nature: Adamant
EVs: 252 Atk/4 Def/252 Spe
Item: Choice Band/Sharp Beak/Sky Plate
Ability: Gale Wings
or
-Brave Bird
-Swords Dance/Will-o-Wisp
-Taunt
-Roost
Nature: Careful
EVs: 248 HP/8 Def/252 SDef
Item: Leftovers
Ability: Gale Wings
or
-Swords Dance
-Flare Blitz
-Brave Bird
-Natural Gift
Nature: Jolly
EVs: 80 HP/252 Atk/176 Spe
Item: Liechi Berry/Salac Berry
Ability: Gale Wings
This team has a nasty Fighting weakness, so I'd consider removing Hydreigon and replacing it with something else that resists Fighting, but I don't know which.
