Blastoise likes hazard support such as Spikes, Toxic Spikes + Rocks because they help wearing down common switch-ins like Florges, Snorlax, Porygon etc. and proceeds to pressure them more. Roserade or Chesnaught fit in pretty well as they give you Spikes support + a grass and/or electric resist which synergizes pretty well with Mega Blastoise. Because Mega Blastoise is already a good hazard removal you can take advantage of that and use something like DD Salamence or DD Gyarados which benefit from Blas removing rocks and the spikes support aswell. And because you want Spikes you kind of want a way to prevent hazard removal :o, so something like Sableye or Chandelure pair well on the latter they form a Fire-Water-Grass which always good to have, hope I helped :)
Thanks man, you've been a huge help. I've taken your advice and picked out some of my favourites (I'm too much of a scrub to use any other Pokemon) for the team. It doesn't have a spinblocker but I'm not really partial to any Ghost-types. Still, I think it's decent. Here's the team if you're interested:
Kahuna (Blastoise-Mega) (M) @ Blastoisinite
Ability: Rain Dish
EVs: 184 HP / 252 SpA / 72 Spe
Modest Nature
- Ice Beam
- Dark Pulse
- Scald
- Rapid Spin
Anafenza (Chesnaught) (F) @ Leftovers
Ability: Overgrow
EVs: 252 HP / 252 Def / 4 SpD
Impish Nature
- Drain Punch
- Leech Seed
- Spikes
- Spiky Shield
Dougal (Arcanine) (M) @ Leftovers
Ability: Intimidate
EVs: 248 HP / 8 Atk / 252 SpD
Careful Nature
- Flare Blitz
- Morning Sun
- Roar
- Will-O-Wisp
Sarkhan (Salamence) (M) @ Life Orb
Ability: Moxie
EVs: 252 Atk / 4 SpA / 252 Spe
Naive Nature
- Outrage
- Fire Blast
- Iron Tail
- Dragon Dance
Claru (Nidoqueen) (F) @ Life Orb
Ability: Sheer Force
EVs: 180 HP / 252 SpA / 76 Spe
Modest Nature
IVs: 0 Atk
- Sludge Wave
- Earth Power
- Ice Beam
- Stealth Rock
Custard (Heliolisk) (F) @ Life Orb
Ability: Dry Skin
EVs: 252 SpA / 4 SpD / 252 Spe
Timid Nature
- Focus Blast
- Hyper Voice
- Thunderbolt
- Volt Switch