tagging users

I am looking for this:

I (or other registered User) writes a Post.

I (or other user) can “tag” another registered user by using @username (or similar)

This “tagged” user receives a (best case) email notification

” things must be possible:

1. When I tag with @username, a list of availabel users must appear (lets say just like the Google +, or Facebook )
2. Some kind of notification must be sent to the tagged user.

Thant’s all. This is also why I do NOT want any kind of BuddyPress or whatever heavy weight add-on.

I think this could even be a great “next feature” for WP itself.

By now, I found this:
Tagged User Notification Plugin.
BUT:
Not updated since 2 years + does not make the “list of users” available on @”start_typing”

Perhaps some body could share ideas or such?

Thanks to the community