My personal guess: there are still two demon generals and a king left (in the current beta) or three generals in the stable version. That means... at least another year until the main storyline ends. And then something more interesting is expected: the expansion of secondary character arcs, which, in my opinion, is much more interesting, as it will make the world of Granoria more vibrant.
Im assuming at some point it will become mandatory to interact with the demoness prior to beating the king as she has scenes talking to the king.
We would still have our quest "become the strongest mage" Although that potentially also get completed upon beating the king.
In which case it would as you said just be fleshing out the character backstories currently present and presumably getting the 4th member for the pirate crew.