For months Canonical has basically admitted defeat with their Ubuntu Software Center "app store" on the Ubuntu desktop. They've been wanting a new software store/center for a few years now and they decided with Ubuntu 16.04 LTS to transition to GNOME Software -- GNOME's software center.
GNOME Software with any Ubuntu-specific plug-ins have yet to land in the Ubuntu Xenial Xerus archive, but the team has now setup a Personal Package Archive (PPA) if you are wanting to help test out this next-generation solution. Robert Ancell announced this morning that there is now a GNOME Software PPA with PackageKit 1.0 and the latest GNOME Software plus patches for the Ubuntu review server. The work is being done against GNOME Git with hopes of upstreaming changes.
More details on the current GNOME Software work for Ubuntu 16.04 LTS can be found via this mailing list post.