The only migration destination I found is UL meta (here). I say this because I saw that SO has five. Destinations I can think for are SO, SU, AU.
Part of the problem is people here don't vote to close. Most posts are closed by a mod and nobody else; in the last 4 months the only votes to close besides mods are Shawn once, Stefan twice, and Gilles a handful of times. Either that means few posts need to be closed on UL, or it means posts only get closed if a mod notices them. If it's the former than that's good, but we're unlikely to warrant migration paths; if it's the latter than we don't know where we need to migrate to, because we can't check the closed posts to find out.
Mods can migrate now, but it doesn't happen as much as you might think. So far this year:
Migrations to us from X
- SO - 38
- SU - 3
- AU - 3
Migrations from us to X
- SO - 3
- SU - 4
- AU - 4
The only one that would really be useful is a target to us from SO, but they don't have room on their close dialog (the max is 5). I looked into replacing Webmasters, since I figured they have the lowest migration volume from SO, but SO pushes around 30 questions to webmasters every week, so having pushed 40 to us in two months is probably not enough to warrant replacing them
You can have other migration destinations, but they must be based on data.
Take a look at your closed off-topic questions and see if there is a pattern of where those questions would belong elsewhere in the network.
If there is strong evidence that a lot of closed off-topic questions belong on a particular site we will unlock that migration route.