The "fantasy" part is just that the girls have different "needs" and problems, it is not like they are going round wielding magic swords.

Open ended? Maid-chan is not sure how to describe. There is no "Happy End now time stops" moment. Life goes on, and it certainly does so for the Demi-chans.

But it is not a cliffhanger or a "Japanese ending". This is not an anime based on light novels which go on for several more books. The story starts at some point in time and ends at some point in time after you learned a lot about the Demis.