The user and question namespaces in the back-end system should be merged such that a question and its answers can easily be "cross-posted" with answers shared across several sites when it makes sense to do so, and such that users are able to participate, search, and interact across the spectrum of SOI sites without need to repeat the same activities on all sites and without overlooking information found on other sites. If that were done, the debate over question migration could now have an option to make the question appear on both sites, and it wouldn't be so bad to have extra SOI sites like Unix/Linux and Ubuntu.
In my vision, questions would almost always still be attached to just one of the sites, and users would still have to sign up for a site to have access to it. User profiles, though, would span across all sites, and users could view the status of their questions across all sites, and could search across all sites to find answers to questions that might be found on multiple sites.
Already, one needed to search for or post the same question in more than one place in order to get answers from everyone active on an SOI site. I can't reasonably bring myself to cross-post to four SOI sites when it's already debatable whether cross-posting is permitted. Of course, creating or searching for separate posts on different SOI sites for the same question shouldn't be necessary in the first place.
SOI, for instance, has succeeded in fracturing the Ubuntu SOI community now to FOUR SOI sites: SuperUser, ServerFault, Unix SE, and Ubuntu SE.
Questions about Ubuntu get answered in SECONDS on ServerFault and SuperUser. On Ubuntu, they often don't get answered at all. Why would the SOI communit(ies) want to expend effort convincing people to come to Ubuntu SE when they're already at SF and SU, asking and answering Ubuntu questions? I'd rather see all of the SE Ubuntu users back on SU and SF participating in the ACTIVE community there rather than fracturing the users among four different sites. As a compromise, though, would it not make sense to let the Ubuntu questions that also pertain to other distros get cross-posted to SU or SO if a 100+ rep user thinks it makes sense? And would it not make sense to merge the answers into a single list?
This need not be something visible to the users, other than the presence of added features facilitated by it.