Our usual practice (for questions put here first that we consider off-topic but think might fit somewhere else) is to usually avoid “automatic” migration to other sites. Instead, we just close them as off-topic and comment with a link to the other site to look into—with the explicit warning that it’s the querent’s responsibility to check that site’s rules and make sure their question fits there, too. We do this because it’s hard for us to remain up-to-date on the expectations and policies of each other site, and it seems unfair for us to shove our mess onto another site, particularly if it won’t fare any better there than it did here.
After all, having a question closed is frustrating and disappointing, but having it constantly bounced around and re-closed is way worse. It’s like waiting on hold with customer service, only to be immediately transferred to another department where you’re put on hold again.
That said, personally, I’m not like, opposed to automatic migrations or anything, or expecting other sites to treat us the same way. In my mind, we do it that way primarily because no one has wanted to put the effort you’re putting in now. So long as there is a reasonable amount of care to avoid bouncing stuff we’re just going to close, it’d be fine, in my opinion.
As for the guidelines, well, huh. As usual for Stacks, our rules about topicality and question handling are, of course, spread around a dozen different Meta discussions. Things that do stick out,
We don’t do recommendation/“shopping” questions. Someone asking for the best system to use to play in SF&F world X is not a question we can handle here—as a question, anyway. You could perhaps point people to Role-Playing Games Chat for that kind of conversation, or to our list of RPG discussion forums which may be better-suited to such a discussion.
We don’t do idea-generation. So even if someone’s got a system in mind, say, something like “I need some ideas for monsters to use in my D&D campaign” isn’t a question we’ll be able to answer. Again, the chat room or the forum link might be appropriate.
We don’t do “why did the authors/designers do things this way?” questions. They just always turn out poorly, with a lot of unsourced speculation and extra moderation headache. We do handle “history of gaming” questions, with the distinction being questions that are less “what were they thinking?” and more “what was the precedent/context in which this choice was made?” In any event, I suspect this kind of thing would rarely be asked on SF&F.
We do answer questions about the worlds/settings of RPGs, which I imagine is the kind of thing that would be most likely to get asked on SF&F—but SF&F handles those kinds of questions, too, as I understand things. Just because we would accept those questions doesn’t mean you have to migrate them—it’s normal for there to possibly be more than one Stack that could handle a question, and “being on-topic elsewhere” does not automatically mean “is off-topic here.” Still, if for whatever reason, SF&F felt they couldn’t handle such a question but it could be handled (i.e. you are the wrong people to answer it), that might be an acceptable thing to migrate here.
We only do this, though, for questions that really have to do with the RPG world. There are plenty of RPGs set in Middle Earth, but questions about Tolkien’s work are largely better off on SF&F, and we would “migrate” such questions that way. But if, say, a particular Tolkien-licensing RPG made some unusual choice for its world that isn’t in Tolkien’s work, either a new invention or even a discrepancy, we might handle that. But usually we just handle settings that only or primarily exist for RPGs (e.g. Eberron, Forgotten Realms, Golarion, World of Darkness, etc.).
The rest of our rules are the usual Stack Exchange rules, more or less, I would expect—questions have to be clear, with enough details to allow there to at least theoretically be a singular “best” answer, they can’t be too broad, they can’t just be a popularity contest or asking after people’s personal preferences, etc.