Hi, I don't think that's a good idea. There might be users who don't want to change their avatar or don't want to use an avatar, at all. So here's what I think would be the best solution:
1. New user created with Nextend plugin: Use the current avatar from the social network. (Don't automatically update the avatar when the profile picture of the social network is changed.)
2.1. Existing user linked: Don't change the avatar.
2.2. After linking process is complete: Add a button beside the "unlink"-button that says something like "Use the current Google profile picture as avatar"
2.3. When this button is clicked: Add the G+ profile picture as avatar, but only once.
2.4. When the G+ profile picture changes, do nothing (do not update the avatar with the new profile picture), unless the user clicks on that button, again.
What do you think about this?