I have a couple of questions for the engineers about these "invention" threads to help sort out exactly what the problem is in order to then figure out what the solution should be.
What is the problem with the invention ideas? Are they ridiculously far-fetched and impossible ideas that border on crackpottery and have no place at PF at all? Or, are they something that might be a good idea, but isn't developed enough to really be at a stage where sound engineering principles can be applied?
If they are totally far-fetched, they probably belong deleted in the overly-speculative post category.
If they are just too undeveloped but might contain a good idea that's not de facto impossible, maybe they just belong someplace else.
In the case of undeveloped but not outright impossible ideas, what types of things are the inventors seeking in coming here? Are they looking for general opinions on whether it is a good idea, something that really fills a need, actually novel, etc.? Are they seeking assistance on developing an idea into an actual design? Or, do they think they already have an adequate design/idea and are trying to get advice on how build it?
If they are just looking for advice on whether it fills a need, is novel, etc., maybe those threads belong in GD.
If they are seeking advice on how to work through the design process, maybe they just need a subforum where they can ask for that and people who are willing to participate can do so, and where it doesn't detract from other engineering discussions in the main forums.
If they think they have an adequate design (and perhaps don't) and are seeking advice on how to build it, I'm not so sure where to put it. Maybe GD again. It might be the same place to put threads from people who have legitimate plans for building something (not a new invention, but plans from some other source for them to just build something) and need advice on how to interpret them, or on sources of supplies, or of an acceptable, safe substitution for a part they can't get, etc.
I can imagine there are a lot of "invention" and "how-to-build-it" type threads and ideas that don't violate the PF global guidelines and are reasonable to ask somewhere on PF, but aren't actually engineering. Maybe I'd even lump them into the same category as the kids working on science fair projects who need help designing some apparatus for their experiment or figuring out how to build something for a project they're doing, or those robot wars contests, etc.
I wouldn't necessarily want those threads placed too far from the engineering forums, though, mainly because the engineers here tend to be the people best able to answer those types of questions, even if they aren't directly engineering questions.
Or, maybe there just needs to be a sticky thread or FAQ thread or library entry that gives a run-down of some basic steps of the invention process and says, "If you haven't gotten to this step..." whatever that step is, "then the topic is not yet suitable for our engineering forums," and direct people posting such threads there. Maybe some resources on how to contact trustworthy sources to work with someone with a good idea to help develop the invention to the next level would be appropriate as well, if giving that sort of advice is beyond the scope of what can reasonably be given here, would be something to point these people toward, and then shut down their thread if their ideas are still too vague to really be discussed from an engineering standpoint. It may just be that they need to know how to hire an engineer to do what they are asking because it isn't something people can just do for free.
Just tossing out some ideas on the different types of issues and some possible ways to approach it so we don't alienate people who might actually take a keen interest in the discussions around here even if the question they are asking just isn't within the scope of the type of assistance that can be offered here.