The name "importer" carries some inaccurate implications.
1) Importers should "import". Tasks like "sync" will do the actual importing. The object only holds the configuration that happens to be used by sync tasks.
2) Sync tasks on mirror mode remove content as well as add it, so "import" isn't quite right.
Proposed name: Remote
The inspiration for remote is "git remote". In git, remotes represent external repositories, which is almost exactly what our importers do.
Part 2: Trim the fields
Currently, Importers have settings that can be categorized in 2 ways. I am proposing removing the "sync settings" from the Remote model:
External Source information
This had some advantages when Importers were related to Repositories. For example, having a repository.importer that always used the same sync mode made sense. However, the "how" to sync settings don't make much sense when importers and repositories are not linked. It seems very reasonable that a user might have 2 repositories that sync from the same source (ex EPEL). It does not make sense for them to have create an Importer for the EPEL repository twice or more just to change sync_mode or download policy. Instead of modeling these fields, I propose that they should POST body parameters.
POST v3/remotes/1234/sync/ repositorty=myrepo_href sync_mode=additive, dl_policy=immediate
POST v3/remotes/1234/sync/ repositorty=myother_href sync_mode=mirror, dl_policy=deferred