A "Google Hangout clone" would, in fact, essentially be a contact management app which dings you when someone wants to talk to you - at the moment, you can use practically any site, they all give you links to share with your friends.

The issue is, of course, getting people to add all their friends into yet another app. You might be able to grab them from Facebook, but then you'll lose all your users as soon as Facebook implements video chat, and you still have to convince everyone to have your app open.