The migrate_plus sandbox is basically serving a place to do POCs and initial development of various migration-related contrib functionality - the replacements for any D7 migrate and migrate_d2d features not going into core in 8.0 (keeping in mind that the most generally-useful pieces might get into core in a point release). It's convenient for now to have them in a single project so as we chase HEAD they can all be updated in one fell
git pull, but once the API stabilizes (see the open API Issues at ) I think we'll be able to start splitting out pieces, which different people could take ownership of. Let's work out how to break it down, and start splitting them into submodules within this sandbox, so they're ready to go out on their own.
Tools for running and monitoring migrations
This would include the drush commands (migrate-status, migrate-import, etc.) now being developed here, as well as a UI equivalent to D7's dashboard. I would actually hope the drush commands get folded into core drush at some point and replace the migrate-manifest command, which is far more limited. Until then, I would say the drush commands and the UI would be in one module - call it... migrate_tools?
We've got a patch now for a CSV source plugin, and XML is a major requirement, plus D7 has support for JSON, SQL Server, Oracle, DB2, Excel, and raw HTML files. Should there be a single module/project holding a grabbag of plugins (migrate_source_plugins), or should each be on its own (migrate_source_xml etc.)?
The basics of the old migrate_example are here now - should they be in their own project? Folded into Examples? Perhaps part of the afore-mentioned migrate_tools?
Note there's an updated migrate_example_baseball in the works as well - I think that would go along with the CSV source plugin, wherever that lands.
Drupal-to-Drupal migration support
Core has basic configurations and plugins for D6 and D7 sources, oriented towards complete site upgrades with no modifications. In contrib we'll want to provide tools to support customized Drupal-to-Drupal migrations as migrate_d2d has done in the past. We'll also want to support migration from D5 and D8. My inclination is to make the tools (UI, plus any underlying support for coded custom migrations) the D8 version of migrate_d2d, and migration from other versions should be separate projects/modules.