If I create a project, and select only a few standard schemas, there is no way I can add more schema later (except if I select the schema files, which is obviously a burden).
Typically, I created a new project, selecting only 'core'. Bad move, as 'core' depends on 'system'
Basically, another way of analysis this request would also means that we should allow the user to select a schema and all the related schema, avoiding such a deadlock issue (ie, if I select 'core' the system should tell me "please select 'system' too". kind of.)