I went for an interview and was asked to suggest a detailed product plan. One month later they deployed that plan and never hired me. I am feeling cheated. How do I ensure such a thing is not repeated with me?”
This is sad but unavoidable. Not everyone would have the integrity, you are expecting out of them. However, when you share the product plan can you not share the key points? Suppose when you tell them about product design, can you not discuss in details? Even if they try and grill you, brush it off saying, you are saving it for later.
There can be no fail-safe method, but a way out for sure. Most importantly, how often do you blog? Can you consider blogging about work? Working-out-loud is a concept which works best for such situations where no one can steal your concept. Finally, an idea to be as good as the one who implements it. That creates a level-playing field between the one who conceives the idea and one who implements it. Try and learn from the mistakes they make while implementing your idea and you have a better full proof plan, if ever you need to work on it for their competition 🙂 Do consider pitching them for sure!
The best part of every problem, is it is never the end of the road. Keep getting better and stronger. Wish you all the best!