Palafin
Jet Punch and Choice Scarf go together as well as flames and an open fuse. A priority move finds minimal use when using an item that increases Speed and locks your move choice. Replace it with Mystic Water, as 3/4 of your moveset benefits from it. 32 in HP alone doesn't tank anything in particular, so it's more beneficial to just put that all into Speed.
Mega Scizor
Knock Off > Bug Bite finds more practicality with item removal and helps Scizor against matchups like Aegislash and Basculegion. 32 into HP gives more bulk for Scizor than 17 in Defense and Sp. Defense, with the rest into Speed or another defensive stat.
Mega Aggron —> Corviknight
Mega Aggron isn't useful, as despite all its bulk, it has to rely exclusively on Rest (which your set doesn't have) to recover any HP. Corviknight does what it can with proper recovery and the ability to pivot out of a poor situation with U-Turn. This also saves the Mega slot for our next Pokémon.
Corviknight @ Leftovers
Ability: Pressure
Level: 50
EVs: 32 HP / 32 Def / 2 SpD
Impish Nature
- Body Press
- U-turn
- Iron Defense
- Roost
Gengar —> Mega Gengar
Mega evolving Gengar allows it to trap opponents with Shadow Tag to force them to stay in, have Speed that doesn't have it rely on Focus Sash, and serve as a better Revenge Killer overall. Focus Blast outweighs Thunderbolt and Giga Drain, as it helps against bulky Dark and Steel-types that those moves can't. Thunderbolt doesn't help with any poor matchups, and Giga Drain's recovery isn't necessary for Gengar, so both moves are redundant. Destiny Bond is to ensure that, if necessary, Mega Gengar takes its opponent down with it.
Gengar-Mega @ Gengarite
Ability: Shadow Tag
Level: 50
EVs: 2 HP / 32 SpA / 32 Spe
Timid Nature
- Shadow Ball
- Sludge Wave
- Focus Blast
- Destiny Bond
Rotom Heat
Rotom needs the Sp. Atk investment, so choose between if you want to invest in HP or Speed. If HP, change the nature to Modest or something to boost defense. If Speed, Thunderbolt > Pain Split so you don't have to force Rotom Heat out of the battle to use an Electric-type move. You might want to consider using Rotom Wash instead to avoid taking more Stealth Rock damage, though it has type overlap with Palafin.
Dragonite —> Garchomp
Dragonite's set is the weakest element on your current team. Fly and Protect do very little besides give your opponent a free turn to switch out, setup, or do whatever they please. Outrage is always preferred over Dragon Claw as, despite being locked into it, the damage decrease is severely noticeable. Dragonite's generally intended to use its bulk to use Dragon Dance and sweep with Espeed and Earthquake. Garchomp is a meta staple in setting Stealth Rock and having the potential to sweep with Swords Dance, while complementing the current team far better.
Garchomp @ Focus Sash
Ability: Rough Skin
Level: 50
EVs: 2 HP / 32 Atk / 32 Spe
Jolly Nature
- Outrage
- Earthquake
- Stealth Rock
- Swords Dance
