In wikitech-l, Ideas for tech-related IdeaLab Campaigns? derives into a discussion about whether initial concepts should be published in IdeaLabs or Phabricator.
There are different places where a project concept could be sensibly published (mediawiki.org, IdeaLab, Phabricator) and there are different processes for prioritization (WMF quarterly goals, IEG grants process, Possible-Tech-Projects, Communtiy Tech and TCB-Team wishlists...). Even if a central location for publishing project concepts might sound a good idea "simpler for those proposing new projects and for those discussing them", in practice there is a risk of disrupting process that are working decently well today and also a risk of alienating the participants of this process. Not a simple question.
However, it seems that we would all benefit from a central log that people could watch to learn about new candidates for prioritization and check to know the prioritization status of existing proposals. Nowadays it takes almost a full time job to follow everything. No idea how this central place would look like or how it should be build, but we could start with a sensible requirement for the proposals pushed by WMF teams...?