I’ve not encountered Sean Howard’s Three Hundred Game Mechanics before. Beginning in June this year (and already up to #58) it’s a collection of ideas for making interesting games. Not the games themselves, but simply concepts or conceits that Howard would be interested to see appearing in games. With the added suggestion that others go make games based on them.
#10: Time Spy. “A turn based game where you can send a spy to previous turns or even future turns.”
#53: Alphabet Rogue. “A roguelike for beginners, where the alphabet is the deciding factor in combat.”