Sites soliciting other types of research and advice (beyond our traditional Q&A subjects) were created in a time when the sheer size of our software developer audience allowed us to give sites like Software Recommendations a shot. Hardware Recommendations later expanded on that format and it has struggled critically with nearly half the questions either deleted or closed. Proposals have since continued pushing out even further into website recommendations, book recommendations, travel recommendations, idea exchanges, essay reviews, textbook errata, job scams registries, software comparisons, suggest-a-song —
I don't have any regrets with the sites we tried, but hardware and software search is about as far as we can take this format; we don't wish to keep expanding this model any further.
I can appreciate that you have [other] questions you would love to ask on a Stack Exchange site, but gathering up questions from a community where they're not wanted doesn't seem to make them work any better here. Honestly, where we should be spending our energy is in figuring out how to make the best of these questions work on our existing sites, but that's a big boat to steer and we need a lot of software support and a buy-in from the communities that host these subjects.
I know it's frustrating to have a proposal closed. I was hoping to come here with a more affirmative statement of the types of proposals that do work, but saying "proposals should be about an area of study (not a type of question)" doesn't quite cover it. Someday I hope to replace Area 51 with "Stack Exchange Labs" where folks can experiment harmlessly with site ideas to see how they pan out, but in a time where we've committed continuing support of the sites we launch, we don't want to optimize for sites that are not well-supported by our format.