As we're coming out of Beta we will need to set up the site's initial default migration paths at some point, and I agree that Physics and Travel seem like good candidates (and really the only major candidates I can think of offhand - Workplace may be a candidate if we get a lot of airline-job-related questions but that seems marginal at best as I doubt those questions would be "migration worthy").
I hesitate to advocate for adding migration paths now though for the reasons Jon Ericson noted in his answer to a previous question on this topic.
The numbers have changed but the logic hasn't: According to our migratio history we've migrated a grand total of 6 questions: 3 to Travel (all open, some reasonably highly voted), 2 to Physics (one at +1, one at -1), 1 to Skeptics (closed as a duplicate).
To me that doesn't scream "pressing need for user-accessible migration paths" at the moment, though it may change as we start getting more traffic.