TL;DR: a non-SE site and I don't know which.
Where ought I to ask/might I get the best help?
Meta or chat?
Not Meta, because that's for discussion about how the site works, not how some other programs shall work.
Also not chat, since the chat rooms need to be on the same topic as the site itself (at least that's what the description says, I doubt it's true for all chat rooms on Stack Overflow).
No. The requirements needed for implementing a software are much more detailed that the requirements for suggesting existing solutions.
Thus, I slightly disagree with @Nicolas' answer. The part "all of your requirements are in it" is simply not true. That would be a 50+ page document which does not fit on this site.
I don't know any Stack Exchange site that has requirements as on topic. I'd like to see "requirements" in an Area51 project. Might still turn out to not fit for single requirements but rather requirements concepts.
A non-S.E site?
Maybe. There are so many, one of them has to support this ;-)
One could perhaps create a Github project for the requirements only. The discussion could be done via the Github Issues. Once the requirement is clear, it could be added to the repository using a diffable file format and the Issue gets closed.
Thoughts: you have traceability of requirements via the Git history. You could implement linking by writing requirements in Markdown and use links.
Once the requirements are done, create a new repository for implementing the application.
Even cooler: if it works like a compiler. As early as possible, write the requirements for the software with that software. Just don't do it like Enterprise Architect, who seem to never have used a UML diagram to design their UML diagram tool ;-)