I'm glad people want to conribute. But everybody has ideas.

You have to realise that "contributing an idea" for developers without any of your own work sounds awfully lot like asking people to work for you for free. That is not going to make you popular in FOSS circles. Most FOSS projects are undermanned as-is and maintaining is a thankless task.

Like others have said, the best way would be to just start coding it yourself. People see you put work into something, they can get more excited about it. Advertising is fine, but unless you have something to show, it's unlikely to attract much attention.

There is a reason "a platform where regular people can suggest FOSS ideas to developers" doesn't really exist. We have our own ideas, which take more time than we have already. A platform such as that would likely be full of people throwing out ideas and close to zero developers willing to work on them.