I was thinking that a cleric would be a good choice for your last slot, to keep up Gyarados and Hydreigon's health and remove status effects that would hamper a sweep. Blissey, Clefable, Florges, and Sylveon are good choices. You could roll with a Spinner/Defogger too if you like, but your team isn't too weak to hazards.
Mega Aggron lacks any sort of recovery, so it can't make good use of Toxic. Give it Thunder Wave so it can parashuffle. It's also best to put EVs into its Sp.Defense, not its Defense. It's Defense stat is so godly that it only needs a +Defense nature to take physical hits well. Filter helps too. Sp.Defense EVs allow it to not be completely destroyed by special attacks.
Rotom-W should swap Thunder Wave with Will-o-Wisp, to soften physical blows from opposing pokes. Burn's a useful status effect.
Nah ンン
“No, I... I have to be strong. Everyone expects me to."