I think we can actually put some theories on the wiki as an archive. Sometimes speculation can be relevant to an article, such as Adonalsium, which we don't really know what it is, precisely. So it's kind of guesswork.
We should not discuss theories, at least, but commonly espoused ideas can be placed in there. I want to make a Theories category on the wiki, with special rules and such for that. My rationale being that if the Coppermind is the complete source, then these things are useful to have in there. But I'm thinking more cosmere ideas, there. Book speculation is something entirely different.
I don't know how we want to handle book speculation. On one hand, if at the bottom of an article people want to put Speculation, and link to various ideas, I think that could be okay, as long as that does not overshadow the factual contents of the article.
Ultimately, anything on the wiki should add to people's understanding of the topic at hand. Can speculation do that, sometimes? Yes, of course. If it is really, genuinely adding to the article, then I believe my only additional requirement would be to ensure that speculation is clearly labeled.
There is a lot of wiggle-room here. We should take things on a case-by-case basis. Darniil made this question on the Talk:Spook page, so I'm assuming this would be about if Spook is the Lord Mistborn. That's a really low-level speculation (and it's heavily implied). Are we seriously going to contest that?
Like I said, I would need to know what was going to be said to make a judgment. If it's about the Lord Mistborn, carry on.