Initial suggestion 
Choking Fungus Cloud
Enchantment
All fungus creatures you control gain "Remove a spore counter: Put a 1/1 green Saproling token onto the battlefield"
As long as a Saproling is in play, opponents cannot play spells.
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Thallids are my favorite deck, but they're just not competitive in Vintage. I think this represents an inherent unfairness in the format. They need something that totally nullifies their weakness of being slow while also making them incredible lock pieces.
I am sick of the fact that Workshop keeps getting more tools to play AND they have the means to power out those tools with the 5 black lotus in there decks. Shop has Mishra's Workshop, Blue has Key/Ring/Mana Vault/Grim Monolith and Fish has things like Noble Hierarch and Aether Vial. Speed wise it just is not fair. And the things they power out can seal the win pretty quickly (Tezz/Jace/Golems/Wires/...)
Ok so, how would you create a card to ratchet up Thallids? I don't think there's any reason that my personal deck choice shouldn't be a tier one option... Suggestions?