Migration paths are only created if a need is demonstrated. It isn't “open it and see if anybody goes through”, it's “there have been many flags and the moderators are tired of dealing with them.” There's no precise threshold, but one migration every month won't cut it. One migration every day would.
Sure, there would be more migrations if there was a predefined path. But not that many more.
There are no migration paths to beta sites, so Bicycles is the only candidate at the moment. (I personally disagree with this policy, but that's how it is.)
Another consideration to take into account is questions that would be wrongly migrated, either because they're off-topic or otherwise unsuitable on the target site, or because they were on-topic here in the first place. I already see a lot of questions being closed because they happen to be about walking outdoors, or about cycling, or about something that an expat might to, even though they are perfectly fit for this site since they are about travel. On these grounds alone I would be very cautious in opening a migration path.