This sounds pretty fun, and I've had a few ideas for it:
Slowbro @ Slowbronite
Ability: Amnesia/Trick Room/Safeguard
EVs: 252 HP / 252 Def / 4 SpA
Bold Nature
IVs: 0 Atk
- Slack Off
- Scald
- Ice Beam
- Calm Mind
This is standard fare Mega Crobro, but at the cost of pre-mega Regenerator it gains a few options depending on what you think MegaBro normally struggles with most: Amnesia to get a free +2 SpD to offset its huge weakness to Special Moves before it gets a few CMs off, Trick Room makes Slowbro a speed demon for a few turns (although if you ran that you'd set nature to Relaxed and Spe IVs to 0) which allows it to use Calm Mind before being hit, or Safeguard to provide a status immunity for a few turns, deterring your opponent from using Toxic to try and stop your sweep short.
Thundurus-Therian (M) @ Leftovers
Ability: Agility
EVs: 252 SpA / 4 SpD / 252 Spe
Modest Nature
IVs: 0 Atk / 30 Def
- Nasty Plot/Volt Switch
- Thunderbolt
- Hidden Power [Ice]
- Focus Blast
Instant Agility is fantastic for Thundurus-T allowing to hit incredible speeds with which to abuse its 145 Special Attack stat from the get go, and Nasty Plot can allow you to break past Special Walls that try and take Thunderbolts, or you can use incredibly fast Volt Switches to maintain your team's momentum. Resistance to priority Flying is nice, as it means Thundurus-T isn't (that) scared of Pinsir's Quick Attack
+2 252 Atk Aerilate Mega Pinsir Quick Attack vs. 0 HP / 0 Def Thundurus-T: 130-153 (43.4 - 51.1%) -- 5.9% chance to 2HKO
Of course, I don't know if my spread is optimal, but that fact that other Pokemon can also gain instant Speed boosts indicates to me that keeping your speed as high as you can would be beneficial.
Manectric @ Manectite
Ability: Magnet Rise
EVs: 4 Def / 252 Spa / 252 Spe
Timid Nature
IVs: 0 Atk / 30 Def
- Volt Switch
- Thunderbolt
- Hidden Power [Ice]
- Flamethrower
Another way to use Electric types, this might be a bit niche as other Mega powerhouses exist, but I like it. Basically, at the cost of Manectric's pre-mega Electric immunity, you can instead gain a useful immunity to Ground for a few turns, removing Manectric's one weakness and when combined with post-mega Intimidate makes it quite hard to take down for an offensive Pokemon.
Another thing I wondered about is maybe unbanning Genesect (now this was some time ago, so forgive me if I'm remembering some of the facts about Genesect incorrectly), as its ability Download isn't really that notable any more, as anything in the game with Access to Swords Dance/Calm Mind/Work Up etc can basically already do that for its favoured offence (and in the case of Work Up, both at the same time). So to put it simply, as much as I hate the thing, I think Genesect should be unbanned as the thing it was banned for can be done by basically every offensive Pokemon in the game now.
Slowbro @ Slowbronite
Ability: Amnesia/Trick Room/Safeguard
EVs: 252 HP / 252 Def / 4 SpA
Bold Nature
IVs: 0 Atk
- Slack Off
- Scald
- Ice Beam
- Calm Mind
This is standard fare Mega Crobro, but at the cost of pre-mega Regenerator it gains a few options depending on what you think MegaBro normally struggles with most: Amnesia to get a free +2 SpD to offset its huge weakness to Special Moves before it gets a few CMs off, Trick Room makes Slowbro a speed demon for a few turns (although if you ran that you'd set nature to Relaxed and Spe IVs to 0) which allows it to use Calm Mind before being hit, or Safeguard to provide a status immunity for a few turns, deterring your opponent from using Toxic to try and stop your sweep short.
Thundurus-Therian (M) @ Leftovers
Ability: Agility
EVs: 252 SpA / 4 SpD / 252 Spe
Modest Nature
IVs: 0 Atk / 30 Def
- Nasty Plot/Volt Switch
- Thunderbolt
- Hidden Power [Ice]
- Focus Blast
Instant Agility is fantastic for Thundurus-T allowing to hit incredible speeds with which to abuse its 145 Special Attack stat from the get go, and Nasty Plot can allow you to break past Special Walls that try and take Thunderbolts, or you can use incredibly fast Volt Switches to maintain your team's momentum. Resistance to priority Flying is nice, as it means Thundurus-T isn't (that) scared of Pinsir's Quick Attack
+2 252 Atk Aerilate Mega Pinsir Quick Attack vs. 0 HP / 0 Def Thundurus-T: 130-153 (43.4 - 51.1%) -- 5.9% chance to 2HKO
Of course, I don't know if my spread is optimal, but that fact that other Pokemon can also gain instant Speed boosts indicates to me that keeping your speed as high as you can would be beneficial.
Manectric @ Manectite
Ability: Magnet Rise
EVs: 4 Def / 252 Spa / 252 Spe
Timid Nature
IVs: 0 Atk / 30 Def
- Volt Switch
- Thunderbolt
- Hidden Power [Ice]
- Flamethrower
Another way to use Electric types, this might be a bit niche as other Mega powerhouses exist, but I like it. Basically, at the cost of Manectric's pre-mega Electric immunity, you can instead gain a useful immunity to Ground for a few turns, removing Manectric's one weakness and when combined with post-mega Intimidate makes it quite hard to take down for an offensive Pokemon.
Another thing I wondered about is maybe unbanning Genesect (now this was some time ago, so forgive me if I'm remembering some of the facts about Genesect incorrectly), as its ability Download isn't really that notable any more, as anything in the game with Access to Swords Dance/Calm Mind/Work Up etc can basically already do that for its favoured offence (and in the case of Work Up, both at the same time). So to put it simply, as much as I hate the thing, I think Genesect should be unbanned as the thing it was banned for can be done by basically every offensive Pokemon in the game now.