That was a clean ending, most loose ends tied up (Ivy’s starless status and Ciel’s existence being made known being made known to trusted people, as well as Sora’s origins), but with just enough leeway for another season.

I am not sure how far the source material has gone and whether or not it is enough for another season or two, but I fear they made the ending this way because they aren’t sure there might be another season after all.