This is related to Close as off topic, flag to migrate, or both?, but it is specifically about questions which are on-topic where originally posted.
The SE family is getting quite large now, and a lot of the time a question could be in scope for more than one SE. For example, a few months ago I posted a question to Biology.SE, which (and this is the key point) was on-topic for Biology.SE, but was felt to be even more on-topic for the relatively new History of Science and Mathematics SE (which I would agree with, although at that point I hadn't come across HSM.SE). It was closed using the 'off-topic' reason and a comment was left saying "I'm voting to close this question because it would be more appropriate on the History of Science & Math Stack Exchange site". At this point the question was already receiving attention and there was no reason to think it would not be answered satisfactorily at Biology.SE.
My question is: If we see a question that is on-topic for the SE it is submitted to (and receiving attention), but there is a second SE for which we think it is a better match, should we be voting to close - as off-topic, or as anything else?
Another example: we now have a Bioinformatics.SE (which has a total of 575 questions), but we also have 795 questions on Biology.SE tagged as
bioinformatics. Should those questions all be closed as better suited to Bioinformatics.SE? (They're too old to migrate now.) If not, why not? Has bioinformatics become off-topic for Biology.SE now that Bioinformatics.SE is a thing?
I could also see a risk that this resulting in borderline questions ping-ponging between sites. For example, I could see this one being closed on History.SE as more appropriate to Biology.SE, and then vice versa, if History.SE hadn't accepted it.
In the answers to the related question I notice a suggestion that you only propose migration if you have a rep of 1,000+ at the proposed destination, since it can take a while to get a feel for what sorts of questions are in scope.
(If anyone really wants to see the question that prompted this thought it's here, but I'm not asking about this question specifically and I'm not pushing to have it reopened - this is purely for interest.)