With Migrate becoming part of the core upgrade process in Drupal 8, the expectation is that all contrib modules with both D7 and D8 versions will be upgradeable via a migration process. One hitch is that the D8 destination site will have access to the source database, and any files managed via the database (file_managed), but it will not have access to configuration that is in code (as pretty much all migration configuration is). So, although Migrate is usually used for one-time site migrations, some people use it in a feeds-like way for continuous imports, and will quite rightly expect that of all contrib modules Migrate should be migratable.

So, to achieve this, we should implement web services by which the destination site (with the proper credentials) can obtain all Migrate configuration. Part of this work should include a general API that can be used by other modules which maintain at least some configuration in code. And perhaps we could also expose core configuration where necessary (I'm looking at you, drupal_hash_salt).

I see this as being a separate module or two from migrate, but I'm not ready to start that yet, just want to get some feedback and get a firmer idea of what it would look like. I imagine there might be some work necessary on the Migrate end though - we need to review and make sure all the necessary information is exposed in the Migrate API and if necessary add some public getters.