I understand what you are saying.
Here are my additional thoughts-
* In a new forum category, I wouldn't abbreviate Errors, Freaks, Oddities exclusively, but instead suffix the forum usage with (EFO) - I know eBay
uses this term and it's generally respected properly as a category for seller's listings. Do listings sometimes go into mint/used yes, but that is to be expected. I don't see a lot of moderator burden from misuse.
* In the forum I might add a sticky thread on what constitutes, topics for that category and what the category entails.
* Regarding backfilling - I am not sure this is necessary. The forum could be forward looking, and any topics that have been historically discussed wrt EFO's, are still searchable and probably applicable historically under their initial forum categories. A Modern mint error discussion would be historically relevant under Modern mint category. I don't think the reward on moving old content is worth the effort if it is searchable. The sticky could also mention "Older topics are searchable within the other forum's categories"
* The number of threads on EFO topics over a period of time is a real concern. I am not suggesting the topic, within existing categories, has been historically active (it probably hasn't), but that may also be due to the lack of a defined category. In fact online, including all other major philatelic sites, there is no forum site aligned with this at all. This could be a bust, or the beginning of something really invigorating to this niche. I don't know, and I'll be honest about that upfront.
* I think the audience can also grow also by notifying the EFOCC and announcing it's existence in their newsletter. I think you would see some growth inward from there and other EFO clusters of folks.
I will also add EFOs are fun. People like to look at them, and many more probably have no idea how awesome some of them look. It could spur new hobby interests. I also think that similar to the topical collecting forum, we can post EFOs per stamp, or per category (space, science, etc) and continually add content that way as well.
I think that the new Scott EFO catalog is coming out. Linn's also regularly posts content. Much of this can be synthesized into new relevant topics, so I don't think there is a shortage of discuss ideas. Do I think it's going to generate a ton of new content upon inception. No. But I would definitely help and try to grow the category.
Is there a downside risk to this approach, or can we attempt a relatively low friction experiment? World precancels is not exactly winning contributor awards, and I think we can grow a following organically.