Here is a tally of the migrations we've made since September 2014, when the Worldbuilding site entered public beta. (Some settling might have occurred during shipping -- i.e. I hand-counted these because we don't have tools to supply all-time numbers.)
- EL&U (built-in option): 79
- Worldbuilding: 15
- English Language Learners: 9
- Law: 7
- Ebooks: 6
- Super User: 6
- Literature: 4
- Academia: 4
- Science Fiction & Fantasy: 3
- TeX: 3
- Meta (built-in option): 2
- Workplace: 2
- History: 2
And one each for: Physics, Aviation, Graphic Design, Freelancing, Health, WebApps, Biology, Stack Overflow, RPG, Open Source, Ask Different, Community Building.
In the last 90 days, we've migrated one question to Worldbuilding and the migration was rejected. (I don't have older rejection stats.)
Worldbuilding is our most popular non-EL&U migration target, and probably more migrations would be suggested if it were a built-in option like EL&U is. It seems reasonable to ask if we can add it.
Update: Tim Post said in this Meta.SE answer that if there haven't been a lot more migrations, we shouldn't be looking for a custom path. He writes:
Look at the last 90 days of mod-only manual migrations. If there are less than 10 - 15 of them, stop - the problem isn't big enough to warrant a migration path.
If more than 10 - 15 questions have been lobbed over, have more than 80% of them been well-received? If so, just ask for the migration path on either meta site. We'll dig into the database as we research it.
If you're seeing 80% or fewer of them being well-received, it's probably not a good idea to set up the path.