Steps to reproduce:

1. Using Sponge's PermissionService, create a new SubjectCollection using collection = service.loadCollection("foo") (name doesn't matter).
2. Create a subject using subject = collection.loadSubject("bar") (name doesn't matter).
3. When a player logs in, run player.addParent(new HashSet<>(), subject.asSubjectReference()) which will return false, meaning it failed to add the parent.

I would like to use a custom SubjectCollection for representing a town's custom-made ranks, otherwise using plain groups would infect tab completion when using LP group commands.

The common storage system in LP doesn't support adding anything other than groups as parents for users/groups, which is why parents with a collection other than user or group is only supported for custom subject types.

Are there any plans to fully support it in the future?

If I'm honest, probably not.

I'd accept a good PR implementing this, but it's not something I have time to work on myself - sorry!