I'm writing this due to the amount of posts and comments I have seen in which people describe how they have tons of seemingly unrelated ideas, and can't flesh out any one of them. I've been plagued by this issue multiple times, until I started mashing together the best of them to create a solid, Frankenstein's Monster idea. Let me add that I'm not a published writer, and this advice is only something I have found to work, and ultimately save me from blocks and dead-end stories.
Now, this doesn't mean that the end result will be something wacky. Ideas, in general, are very flexible. Seemingly unrelated ideas can be quite perfectly related with minor tweaking, such as setting change, or character motive.
Idea one: In the near future, a woman rides a train through a military controlled America. She ponders upon the beauty and horror of life, while seeing towns burn and people executed right outside the window, passing along like glimpses into the past. Before reaching her destination, the train is attacked, and she is thrust into the world of a rebellion group.
Idea two: During the 1920's, a writer with no family volunteers to be one of the first 500 citizens of a new, underwater city. The city is separate from all other worldly governments, and starts off as a utopia. Everything seems ideal, until the first murder happens in the city, and the writer, obsessed with finding the killer, uncovers the dark underbelly of the people who are running the city.
Mixing idea 1 and 2: Take some time to think how you would do it. I like idea two's setting much more, so I'll go with that for Idea Final. I could combine the characters of 2 and 3, but I think they both have potential. 1 is more passive, and is forced into a life of fighting for a cause. 2 is aggressive, actively putting them self into the pursuit of the evil. Their similarity is their morals, a common existential cause; wrongs should be righted.
Idea Final: The fate of a man and a woman intertwine, as they become two of the first 500 citizens of a new underwater city, utopian and separate from other worldly governments. Then, the first murder happens. The main characters have become friends, and agree that they will, in secret, find the killer. However, when the man disappears, the woman suspects the worst, and goes on the hunt to find the killer, and her friend, all while uncovering the dark underbelly of the people who are running the city.
Conclusion: This method works for me, I hope this post will help someone out there with the Too Many Ideas problem. Let me know of your suggestions, or what you think of this method. Much love, and have a great day/night everyone.