Hydra? How exactly is the theme of Hydra comprehensively covered through the amount of Card Ideas in The Armory?
Instant-kill.
What? Lithification? I have no clue what you're talking about... 
Taking control of enemy creatures.
Rewinding permanents or reseting timers on temporary permanents.
Removing cards from the hand, deck, etc.
Single card with a large or game-breaking effect. (Win the game, wipe the field, switch HPs, etc.)
A card whose mechanic only works in combination with another card, aka Yu-Gi-Oh! archetype cards.
Cards that do not fit the Element they are in.
Themes that do not fit in the game. (Humans, etc.)
In hindsight, as many--if not all--of these mechanics have been showcased (with proper limitations, of course) in perfectly balanced Card Ideas, so I would argue that there is no list. The ones I listed may be good for telling new members what mechanics to avoid without getting a thorough sense of what has been done and the status of the meta-game, though.
