Pretty cool picks, the overall team looks good albeit extremely tailored for singles.
I can see the general idea with the classic Hippo lead -> hazards -> setup -> sweep. Personally not a huge fan, but if it works for you by all means you should keep rocking with it! Preferences aside, though, 5 out of 6 mons focused on setup seems a bit redundant and you could probably use more flexibility in the form of speed control or terrain interactions.
As mentioned by @Siv, you could drop Heat Wave on Naga, Thunder Fang on Hippo and Hurricane on Volc. Instead you could try additional coverage as he suggested or go for some utility: Struggle Bug (Volc) or Snarl (Naga) for debuffs, Tailwind (Naga or Volc) for speed control, Roost (Volc) or Slack Off (Hippo) for recovery, Light Screen on Volc for durability, Sunny Day if you need to get rid of your own sand, even cheeky stuff like Double Team, Substitute or Gastro Acid to screw with your opponent's abilities. Experiment and see what works best!
Fake Out on Kanga also looks very good (maybe drop Sucker Punch, even if it sounds like blasphemy?), it's free chip damage in singles and helps your setup strategy in doubles, plus if you change the ability to Scrappy you can even use it on ghost types before mega evolving! Other than that, Thunder Wave on Gyarados? Even post gen 6 para is still broken, so might as well try it.
As for items, not sure. Razor Fang on Gyara to boost Waterfall flinches seems fun, but it's likely to be useless if you are planning to just sweep with it... At that point just keep your water boosting thingy, or switch to Muscle Band/Expert Belt if you wish to take advantage of the other moves as well. Or totally go the opposite way and slap a Waterium-Z on that bad boy, Hydrovortex your way to your first Moxie kill and start snowballing from there, what the heck.
Hope I gave you some ideas! And if everything else fails don't worry, you always have an auto-win button on Aegislash, lmfao.