Much like the Community user already does with duplicates where a comment is auto-added linking to the duplicate. If a vote-to-close is given for a question belonging to a different Stack Exchange site could a link be added in a comment saying something like: "This question might be better suited on x".
What is meta?
- Any Stack Exchange user can ask a question
- The community provides support, votes on ideas, and reports bugs
- Your voice helps shape the way Stack Exchange operates
Maybe; but 10% of the time the first Off Topic close vote is for the wrong site or the question is crap and shouldn't be migrated anywhere anyway.
I have no data to back this up, but I've seen a few cases of people saying "oh you should use [other site]", then the original questioner copy-and-pasting their question on the other site, and then there's a pile of cleanup work to be done to close with comment or migrate then close as duplicate...
I've personally stopped leaving comments about other sites most of the time and just vote to migrate on the theory that a single post migrated once is easier than questioners duplicating their content on multiple sites.