@ashs If you are serious about your idea, instead of discussing it, you should be ordering a market research for the niche you are considering.
The market research is for the niche or for a generalistic service, so it doesn't need to disclose 100% your idea.
With this market research made, you can get a general view about the viability of your idea.
Also, asking for feedback on an idea in any public forum is... bad idea. Not because they're gonna steal it (they probably won't), but because any niche within a platform caters to a profile of users. Most of these users will have some things in common, including thought patterns. Hence, most of the responses you get will be skewed in one direction or other, and won't provide a full view of the full array of possibilities of the idea.