Nice build. Having access to every pokemon makes you able to create any kind of team. But even I can say your team is pretty balanced, having Naganadel, Alakazam, Kangaskhan as the offense, Seismtoad and Ferrothorn on defense, and Aegislash being and edge of both, which, I assume it's your Ace. Recomendations:
Aegislash shoudl stays as it is imo.
Getting Alakazam as your Mega, as you said it's insane Spd, and Sp.Atk are greatly boosted. Plus, Trace can be useful in E19 battles. As for its 4th slot, I would replace it with Grass Knot.
Having M-Alakazam would make useless Kangaskhan imo, this will open a slot in your team. You may add a Fire type pokemon, why do I say this? I believe every team should have a Fire type, when it comes handy sheer power, which usually Fire types are needed. Good options: Arcanine, Blaziken, Infernape and Volcarona are great choices.
Offensively talking, Swampert is better choice than Seismitoad, though lower in Spd. it best Seismetoad in Atk, Def, and Sp.Def. Having acces to Earthquake.
Access to all pokemon, which means you can always rotate pokemon. Umbreon is clearly an option if you don't want to die. Garchomp is always considered a threat. Assault Vest Tyranitar with +Sp.Def nature and 252 Ev on Sp.Def and HP, is a tank. A-Ninetales with Snow Warning, Aurora Veil and Blizzard, holding Light Clay and many more.
Similar to my Naganadel, Timid, but mine Spd is kinda meh.. Choice Specs/Life Orb hits hard. Nasty is useful if Beast Boost raises Spd, but if yours raises Sp. Atk. then, I woudl take out Nasty plot and add Thunderbolt.
That's all. THIS IS ALL MY OPINION.