Okay, neither of these seem like they'd be particularly hard.
Mysterycraft 1: Literally a 'mysterycraft,' when downloading this you don't know what the materials are. They still have to LOOK like whatever they are, but you won't show us the exact. Why, you may ask? Because the best part of Minecraft was when you weren't an expert. To have to re-learn the elements will let us partially relive that.
Mysterycraft 2: Saw the other title and decided, "HMMM!"
Why not make a texture pack that brings the feel of a murder mystery. Bloodstain clues, zombified victims, shooting murderous people who look vaguely like skeletons with revolvers who aren't really skeletons, specially enhanced spiders, belonging to the murders, exploding rockets that chase you, cow witnesses, rich people who walk on their hands and feet and drop their clothes if you punch them, suspects made entirely out of pork, and chickens. Dungeons could be murderer hide-outs!
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Things I wish I could do: Undo everything I ever did on the internet before the year... I dunno... 2013