If it's free and open source and it's also software, it can be discussed here. Subcommunity of Technology.
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Honestly the best way would be to start coding by yourself. While trying to find solutions you might find the right people too.
Almost every dev has its own ideas and ideals. There is no lack of ideas but everytime a big lack of time and men power. Software developers have more too much on the plate then too less.
So sharing ideas is nice but contributing is gold.
I would call it the FOSS Dev Paradox.
Why not set up a simple flarum instance? I got one running. If OP can drive traffic to it and actually bring developers to it, I'm more than willing to grant OP mod status or even a hestiacp account
I'm afraid it doesn't work this way. Developers generally don't have a shortage of ideas or problems to solve. People suggest ideas all the times. Usually they have no idea if it's super difficult to implement or already a solved problem. And if people do the programming in their spare time: They need to be involved or personally motivated somehow. So you need to find people who also want it.
My advice is: Find out where those people mingle, who would have some personal motivation or involvement with your topic of interest. That is the right place to ask. My personal oppinion: Feel free to also spam the internet and places like this with your idea. I'm a proponent of "Don't ask to ask, just ask". People can always not read your post or can guide you into some direction. It's probably okay if you do it a few times too many. Just don't ask in a hundred places at once and then don't read the replies. If you're better than that, you're fine.
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.
AFAIK, nothing focused exclusively on "giving ideas". If your idea is for free software, there is nothing stopping you from just writing it here and letting people talk about it.