just a quick question, how strong is the API? I will have a quick look at it later this day, but maybe someone can give me a short description.
Example: Would I be able to change the way things are shared by building a plugin? Would I have access to the friendslist, using the API? Would I be able to change the webinterface, adding buttons and stuff?
I'm not sure what you mean by change the way things are shared, but as far as I know there is no API call for retrieving the friends list or modifying the plex/web interface (which is what I believe you're referring to). You could write a userscript or browser extension to modify the web client on the user side though.