With the rise of more scientific sites; such as Physics, Chemistry, Biology, and Computational Science, I find myself wondering where I should post about using scientific software for biophysicists.
Along the lines of the main-site cross-post idea; I propose some mechanism to allow users to perform cross-meta-site posts. This would increase awareness among users that may be interested in the outcome of a given discussion, even if it's happening on another site's meta.
I think status quo now for anything non-trivial is to notify the other site and point back to the original discussion, but this could fragment the thread (people posting and commenting on two, or more, sites), or not involve the other community until later in the process. One post to Rule Them All™ seems desirable.
Here are some awful ideas on this theme: It could be restricted to it's "neighbors" (e.g. the sites I listed in my example, which are perhaps the known migration paths), or simply open to everything. MSO could be everything's neighbor (or not, to reduce noise). Maybe it could work like a fake reverse-migrated question that still shows up on the "source" site.