Seriously? My mistake. I'm not quite sure what could've been created in that case, perhaps a new type of enemy we haven't seen in the Kingdom Hearts universe yet.
And that could open up a window for the new kingdom hearts 3 if anounced.
Hmm may be old news and all but i have an idea (may be completely wrong but who cares). Alright we all know riku gave his heart to the darkness he may have not have become a heartless but he did have a dark form (Haven't looked into it yet or played Ventus's story, but so does vanitas). Also Ansem the seeker of darkness did take over rikus body. And when riku realesed the dark power within his heart he became Ansem the seeker of darkness. So is it possible that this new secret boss can be rikus or riku/Ansem the seeker of darkness/ Xemnas's nobody?