The migration dialog offers meta on all Stack Exchange sites. That's because some users aren't aware that meta exists and post meta questions on the only site they know, which is the main site.
Other sites may be offered but they have to be activated by Stack Exchange sites. This would only be done for sites that we often migrate to. In the 2½-year history of the site, we've only migrated 12 questions¹, so no site comes close to being worth adding to the list.
There has never been a write-in option in the migration dialog. You may be confusing it with the close → off-topic dialog, which does have a write-in option.
If you think a question should be migrated, vote to close as off-topic, and flag it. Moderators can migrate a question to any site. But the question has to be less than 60 days old. Note that questions are only migrated if they meet all of the following criteria:
- Off-topic here.
- On-topic on the proposed site.
- Sufficient quality so that it wouldn't be closed on the target site. Note that the question must be ok in its current state. If the question needs additional information from the asker, just vote to close and suggest that the asker reposts.
¹ You can see this information under the migrated tab of
/tools. To access
/tools, click “review” and select the “Tools” tab. This requires 10k reputation.