Ah, the eternal question of a writer. Cursed with so many ideas and so little decisiveness. I recommend you write out all your brain has on each idea in separate documents and then ask yourself some questions.
1. Which idea is more current? Which would people connect to more? Look at your themes and draw parallels to your world and the real world. Which idea has a message that needs to be said? If there are already books very similar to yours out, you should choose your most unique idea. Alternatively, make sure that that idea has a solid target audience.
2. Which idea is more plotted out? If you have character or a world fleshed out, this is the one you should build on this. As you work on this idea, you can build on your other ideas on the side. The more plotted out a story, the more passion. The more passion, the more motivation. The more motivation, the better the finished product.
3. Can you combine ideas? This is my favorite thing to do. It’s a great exercise if nothing else. Take two of your ideas and see how you can compress them together and how those ideas would bounce off of the other. Oftentimes your end result is a really creative and unique idea that makes two parts a whole. Even if you end up not going with the mashup, it will help you know more where you want your story to go and which idea you’re more willing to sacrifice.
4. Which excites other people the most? Talk to your friends, whether readers or writers. Tell them about your ideas. See which they connect to and encourage you write. Which do they want to read? They’ll likely be the ones giving you feedback as you go along. Why not start now? It helps to bounce ideas off of people. Sometimes even just having someone pick one for you will make you realize which one you really want to write.
5. Which idea is best suited for you? Which one plays to your strengths? If you’re a first time writer, I recommend starting with a one off book instead of a series as it’s a lot easier to keep track of and plot out. For example, if one idea is set in your hometown and the other a place you’ve never been, you can add interesting and deep depth to one that you can’t the other.
Even though you only can (should) pick only one, having all the ideas in a document will help you continue to build on these ideas. While you might only be working on one actively, you can be working on the others passively. Hope this helps 🙂 Go write!