Now that we're out of beta, will we get access to the full set of stackexchange sites for migrations ?
To give some insight into why this probably isn't needed (at least right now): we have very few migrations, and they are “all over the place”. Among 20 questions migrated (away from Academia) for the last 3 months, the most common target was CrossValidated, and it accounted for only 3 migrations. Then we have our own Meta site (2), then all other sites have had only one (successful) migration.
Unless it is very obvious, each migration path is typically discussed separately. SE also requires in most cases that there is already a bit of a history of successful mod migrations to that specific site before enabling a path for community migrations.
So I'd start by looking at the migration statistics, I think only mods can see those, and see if there are any sites where a migration path might be useful. Then you can post a feature request and ask someone from SE to take a look at it.