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.
Software development is a huge amount of work and the idea makes up maybe a 1/10 of a percent of the whole work. Every dev has more than enough ideas, what they are lacking is time to develop all of that.
Telling a dev about your great idea is usually similar to going to Mercedes and telling them your great idea, which is that they should make a green Ferrari with six wheels and a cardboard roof.
a) They got enough to do, they don't require additional ideas
b) Ideas from non-devs are usually super high level ("make a Ferrari"), leave out anything that would make them worthwhile but instead include a lot of unfeasible details
Don't take this offensive, it is not meant offensive.
But I have had so many people (mainly those with a business degree) tell me about the great ideas they had on the toilet. Most often with a "if you implement it, you can have 30% of the profit". And of all of these ideas that non-IT people told me, I have yet to hear one that would be worthwhile implementing.
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.