Ideas and concepts cannot be copyrighted. If you post your ideas, there is always the chance that others that find your ideas will find them appealing, borrow some parts of your ideas, be inspired, or rarely, even rip it all wholesale. However, this is really a negligible worry, since being inspired by parts of other ideas is extremely common, and as for ripping completely, most people are only interested in developing their own ideas and stories.
To top it off, even if someone did steal another's ideas completely, Sturgeon's Law
is still in effect. It will most likely be subpar, especially so if they're completely copying detailed ideas from another person. It is very largely execution that matters in terms of the quality of a product's end result, so I wouldn't worry.