Since this is (allegedly) going to be a darker story, the new composer could use Liszt's technique of thematic transformation to transform previous themes from the Divinity franchise and make them familiar, yet wrong (darker) somehow. Even though Kirill's pieces were already quite dark :p Apocalypse is right though, Kirill did set quite a high standard for the music and just randomly picking a composer who will compose the score using a generic template in an afternoon will decrease the quality of the overall game.