The packages systemd 226-1 plus dbus 1.10.0-3 now launch dbus-daemon once per user; all sessions of a user will share the same D-Bus "session" bus. The pam_systemd module ensures that the right DBUS_SESSION_BUS_ADDRESS is set at login.
This also permits dbus-daemon to defer to systemd for activation instead of spawning server processes itself. However, currently this is not commonly used for session services (as opposed to system services).
kdbus will only support this model, so this is also an opportunity to iron out some bugs and make a future transition to kernel buses easier. Please let us know of any issues.