I'm just spit balling this out there right now (this'll likely grow in answer in the future when I'm at a desk), but something I'm supportive of, and I've heard others are, would be migration between sites should be permissible on old and good quality content that's (no longer) on-topic for a site.
You discuss closure being a problem, especially for old content that is getting reviewed, but no longer on topic. Rather than changing what is allowed, why not allow the community to suggest migration? This could be something moderators take on board for actioning for old posts (but the community can flag for the migration), from both sites, but migration to a different site where something is on-topic, rather than closed and can't garner further answers, seems like a win to me.
As example I closed this question as part of my normal usage of the site (I had searched for a problem I was encountering). The post is old, and clearly off-topic for Stack Overflow but I suspect would be a good candidate for Superuser.
I admit, this could be a slow process, but still worthwhile if well implemented. It would, of course, need support of the mods, as bad migrations are also a poor experience for both sites.