I would suggest adding some Overgrown Arches or some Phantom Nantuko's. The Arches will gain you life and the Phantom is similar to the Hydra. I would also think about adding Nylea's Presence so you can take advantage of the swampwalk and islandwalk regardless of what color deck you're playing. If you're looking for a good resource to search for cards based on any kind of criteria try scryfall.com, their advance search feature is pretty robust, and cards are updated regularly.
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While looking through your creatures I couldn't help but notice that Demon of Catastrophes really doesn't synergize with the deck as everything else is a Zombies or makes Zombies. Might I suggest the following; Falcon Abomination cast for 2 and a blue, it's only a 2/2 but you get a 2/2 zombie token w/decayed as well; Aven Eternal which has the same cost, power, and toughness as the falcon but you get to amass 1 (give a zombie army token +1 or make one and then give it +1); Or for 1 mana more than the demon you can cast Hordewing Skaab it's a 3/3 and all other zombies get flying.
Thanks for the feedback I definitely need to invest in some Guttersnipes and I like the potential synergy with Stoke the Flames
My 2 cents... I agree that adding the Galvanizer would be great for some nice infinites, but I would also add a Myr turbine so that you could fetch your Lodestone or Battlesphere once you have that online.
You should think about running some Immerwolfs. This way once your werewolfs transform you can keep them transformed no matter how many spells are cast.