I appreciate your feature request, although I currently have zero personal interest in the implementation of such a feature.
I would instead like to comment on your example,
"For instance, I think that History of Science And Mathematics and Mathematics Educators are harmful, because they fragment the mathematics community and build overspecialized sites, and I would prefer that all their questions were moved to Math.SE and MathOverflow instead, maybe combined with better filtering capabilities."
While I, a fellow mathematically minded individual, do understand that very small numbers are not identically zero, I also use reasoning and my estimation skills to determine if something is numerically significant.
For example: By my estimation skills, and I could use SE Data Explorer to figure this out if I really cared, the number of posts on either MO or MSE for either the math history tag or any tag involving math education are less than or equal to 1%, and I think I am being gracious there.
If your estimation skills bring you to even a roughly similar numeric conclusion as mine, I implore you to consider how important these newly proposed sites are to the people who have decided to try to make them. Is this really "harmful" to the pre-existing sites? Would such proportions really cause anything more than negligible "fragmentation" for the pre-existing sites? Are the other two sites at any sort of tangible loss considering that any user from MSE or MO can easily join either of the two new sites?
Finally, I completely fail to see any credibility behind your assertion that a community of Math educators, or Math/Science historians is "overspecialized". Both, "Math Education" and "The History of Mathematics" for example, are actually degrees in and of themselves, and very important ones in my opinion.
In the end, I hope that you come to appreciate the enthusiasm of the people who really find these new sites to be important. I implore you to find a shift in your paradigm. Join these sites instead, and help to make them better with your own knowledge. That, in my opinion, would be a great conclusion to this story. Your heart and participation would certainly be a gift to either site.