In developing a novel about ancient Egypt, for instance, keep in mind the audience for that kind of historical fiction rather than the readers of serious archeology. Sometimes it is helpful and necessary to test the remaining possible ideas before making a final choice. Develop each idea further by taking notes about how it could be worked out, compare its potential and interest with the other ideas, and consider how it might help achieve your goals. One idea may be a lot of fun to write but have a very small audience. Another idea may be strong but not as strong as the other remaining ideas.
Time can also be a factor.
Such projects will likely require a significant amount of learning, in research or writing or both, that will extend the completion of the manuscript well beyond the deadline. Finally, choose the most likely idea. This choice does not mean a final commitment but simply a decision to move the idea to the next stage. If the idea continues to work, develop it through the succeeding stages. But if at any time you exhaust its potential or lose interest in it or realize that it isn't nearly as exciting and challenging as you'd thought, set it aside and start conceiving and developing new ideas.