I've tried a few mystery worlds:
1. Pyramid Puzzle
2. First Contact/First Murder
3. And one in which your Dop is a detective in a big city who's suspected of killing a cult leader.
#1 succeeded, but was not fully developed as MJ said it needed more variations. @2 and #3 were not fully developed. Let's see if four times the charm.
I'm going to follow my advice to John. Make it simple. There are only going to be two apparent suspects. And I'll try to make variations in which A and B are guilty. But I'll try for some more variations as well.
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In New Amsterdam, the capital of Her Majesty Queen Victoria the Second's American Provinces, there is a certain park, the Royal Park which is known among the cognoscenti as Unicorn Park. One of the last Unicorns in the world, a beast from the Age of Walking Myth, lives in its house/corral inside the Park. She has two keepers, one John Patrick Morgan, a good Irish lad, and the other is the Verser's Doppleganger or Other.
In all the Variations, the Dop is cursed by a death curse. He sweats, he sneezes, he goes to the pot all too frequently. He's dehydrating to death. But as he collapses, he pulls out a gift from his great-grandmother, a noted witch. Its an amulet about his neck that will Call for Aid once.
The Verser arrives to see his Other on the floor of the kitchen, in desperate need of water. The Other will be delirious, but will not want to touch the Verser.
If they touch, the damage to both ranges from One fingertip (Annoying) to Fireman's Carry (Fatal). This effect does not go away. [One effect in game will be to make some players distrust their Other which is no bad thing.]
If the Other does not soon get water, he will die. If he gets water, he will begin excreting it at a faster rate. This cycle of rehydration and dehyrdation will speed up [It will be somewhat unpredictable. Give the player an estimated time of a week, and then shorten it by suddenly bumping up the dehyrdration speed. This will establish a ticking clock, but also allow the GM to alter it for dramatic effect.]
If he receives enough water, he will be weak, but not delirious. Every minute of conversation is a simple stamina check vs. passing out.
"Someone has put a death curse on him. The only way to relieve it is for the would-be murderer to be unveiled."
1. Their are more effective means of killing someone, but this method is particularly horrible. This implies a great deal of hatred toward the Other.
2. The curse requires a range of less than a hundred feet to the target from the caster. There are no delayed effect spells.
3. The whole Unicorn House complex for the Unicorn and for its two keepers is a house, a barn, and a corral all in one. At night, it has a very strong protective spell prohibiting entry from outside as the Unicorn is very valuable. This spell extends two hundred feet from the center of the house, and over one hundred fifty feet from the bedroom of the Other when he was stricken.
4. It may develop that Other is a fairly strong mage as he knows all the data in #2.
Variant Murders:
1. Irish is angry because he has a girlfriend who wants to be married, but Other per policy refuses to have a married couple around a unicorn. Irish is not a magician of note, but his girlfriend is. He lets her in, and she curses Other. They intend after Other is dead to be the bosses, and hire a celibate animal handler to take care of the unicorn directly.
2. Irish is a magician of enough skill to buy a curse spell from the Cursorium, a magic shoppe on Battery Row, and he uses it on the Other. But he does not intend to kill the Other. He knows that the Unicorn's horn can heal the curse, but he figures this will weaken the unicorn enough that Irish can kill it, and take its horn, and sell it for lots of money to pay his gambling debts.
3. The Unicorn is secretly intelligent, and he's been offended by Irish's rude behavior too it. So he cast a spell, but he messed up, and hit the Other with it rather than Irish. Now he wants to save the Other, but not so much that he will expose himself as intelligent because he fears what the humans would do to an intelligent beast as they had a tendency a few decades back to slaughter them such as the Talking Geese which got wiped out.
4. The Other is a magician, but he has a dark secret. He's consorted with Powers he would have been wiser to let be. And one of them has slipped a servant into his mind, and so he has blackouts. During one of them, he cast the spell on himself.
5. The great-grandmother, a noted witch, foresaw that her great-grandson might meet an immortal verser if he came near death. So she cast a spell (a delayed effect spell, but it turns out that really powerful mages can do this.) and her great-grandson came down with the curse, and summoned the Verser. Now all that is needed is for the Verser to touch the amulet. If he does, he gets dumped in the Dops body, the Dop gets disembodied as a ghost, and the great-grandmother gets an immortal body to use.
And the problem of fighting over the body when the body verses is going to be tough, because the evil, nasty great-grandmother witch is a really hard-headed and cold-blooded woman with a steely will.
NOTE: Those are the variation plans for the murderers. Now I need clues to match up to each of them, but thats a post for later.