Will be ever possible to add a course inside another cours, just like another activity?
How strong is any design/technical constraints to prevent it?
Another Moodler [Rudy Scott] a while back attempted to create a "Subsection" module. But I do not think it was ever fully developed. You can check it out at the following links. Remember he was working with older Moodle code. Maybe v1.3 or v1.4.