I was at a workshop and someone had a few, maybe six pictures of a project they were working on. I said to myself, "I could do that so much better, maybe I will." But nothing has come from it since. Mostly because the idea didn't originate from within and so I didn't feel close to it. It has been in my mind but it is safe to say that nothing will come from it.
I came to realize that the best photographs come from things that are close to you. It might be a place or an idea, but it is something that you are emotionally connected to.
So, from that standpoint you shouldn't worry that other people will photograph your ideas, because no matter what, they will not make the same pictures that you would.
You are right, what if someone beats you to something specific and they have more time, energy, and money and they put a big show together and get a book published? You then come around to complete your project, but by that time you are influenced by what they did, and no matter how good your pictures might be the public will see them as emulative.
I generally don't guard what I am working on or what I plan on photographing, but I don't announce the specifics to the world either. If I am excited about new pictures I am making, or something new I am working on then I talk about it with friends. Some of whom are photographers. They might just steal my ideas, but with that in mind I usually work harder to get it done first.