Great ideas Chiara. We actually do some of them in our social eReading Apps at www.readcloud.com which is used in schools across Australia. As you rightly point out, no-one wants to see annotations from users they don’t know which is why we ring-fence annotations to match school classes. 
You can also add videos inside the eBooks as you suggested and we’re working on some of your other suggestions. Let us know if you would like to check it out.