When users vote or flag to close a question that should be migrated to another site on the network, they usually can only choose to migrate to meta:
Occasionally we get requests to add a new migration path, which can only be done by employees. Typically we reject such requests because migration is often misused. It's also possible for moderators to migrate if notified with Flagging > Closing > Off-Topic > Other. But sometimes there are sensible migration paths and we do create them. Here's my evaluation process:
Sanity check: If there's zero overlap in scope and very little opportunity for honest confusion, then the chance of a good question being asked on the wrong site is small...
- Lego Answers probably doesn't need a migration path to Seasoned Advice.
- Just about every site on our network occasionally gets programming questions, but most don't need a path to (or from) Stack Overflow.
- Brand new sites that have not yet solidified their topic space should not be the target of a migration path.
- Sites that get very few questions should not have outgoing migration paths as a rule.
If a proposed path seems blatantly unlikely to result in productive migrations, then we politely decline the request and recommend flagging a moderator for those occasional exceptions.
Gather statistics: You can find out how often a migration path is used by looking at the 10k tools: https://stackoverflow.com/tools/posts/migrated/stats, for instance. The "away" migrations are the important ones. If a site doesn't have many or if they are often rejected, that's a reason to not create the path. (And yes, this is a chicken and egg problem. Once again, it's always possible to ask a moderator to migrate questions with a custom close vote or flag.)
There's no "right" level for establishing a migration path, but tens or hundreds of migrations with a single digit rejection rate seems safe. Single digit migrations in the last 90 days and/or rejection rates that can order a drink without ID are signs that the path will be a waste of time to establish.
When in doubt, we reject requests for new migration paths because they are often a source of conflict between communities. Readers of this meta are likely to understand both the target and source sites well enough to reliably suggest migrations. But that's not true of all users.
Is there a better (ideally objective) way to evaluate migration path requests?