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Sketch out both (or multiple) ideas as fully-fleshed plots from beginning to end. Get all your separate possibilities down on paper. Put everything aside for a week. Come back to them and re-read...
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#4: Attribution notice removed
Source: https://writers.stackexchange.com/a/21887 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#3: Attribution notice added
Source: https://writers.stackexchange.com/a/21887 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
Sketch out both (or multiple) ideas as fully-fleshed plots from beginning to end. Get all your separate possibilities down on paper. - Put everything aside for a week. Come back to them and re-read them. See if any one jumps out at you. - Give your multiple outlines to friends and see if any one is particularly popular. - Write a series of short stories playing on the very idea that this is the same plot with multiple endings. - Write a literal Choose Your Own Adventure book. If you can't decide, let the reader do it.