You may be confusing pull requests with feature requests. A pull request is when somebody has written some code/functionality/fixes and is asking the maintainers of the project to include (pull) said code in the main branch of the project’s repository for everyone to be able to use.

i.e. if you want a new feature, you make it or convince someone to make it for you

Just in case, maybe you or someone confused what pull request is.

Pull request is for someone who want to contribute by writing some “code”, they submit a request and wait to be pulled by the project maintainer (in this case is the core lemmy dev).

If you want to request for something to be added/fixed, i.e. adding new features, you can open an issue on the github repository.

My hot take (and probably shitty opinion) is that the barrier of entry will filter a lot of the noise.
Those that have the ability to directly contribute already know how to do so, know how to see what’s already been raised, see what’s in progress etc.
GitHub centralises the issues, has full tracking of the processing of those as well as any relevant discussions.
Anything on the fediverse for that would need a lot of work to keep it reconciled with GitHub activity, and would more likely just be a place to placate users.

Once this influx of users cools a bit, once the development can slow a bit, and everything left are big issues… Then yes, I think somewhere for everyone to discuss and get involved is a great idea