"In love did God bring the world into existence; in love is God going to bring it to that wondrous transformed state, and in love will the world be swallowed up in the great mystery of the One who has performed all these things; in love will the whole course of the governance of creation be finally comprised." - St. Isaac the Syrian
"God’s love is so complete in itself—he is lover, responding beloved, and union of the fruit of both—that he has need of no extradivine world in order to have something to love. If such a world is freely created by God, apart from any compelling need, then this occurs, from the viewpoint of the Father, in order to glorify the beloved Son; from the viewpoint of the loving Son, in order to lay everything as a gift at the Father’s feet; and from the viewpoint of the Spirit, in order to lend new expression to the reciprocal love between Father and Son. Hence, the one triune God is Creator of the world. If this creation is attributed specifically to the Father, then that is because, within God, he is the Origin behind which nothing more can be sought." - Fr. Hans Urs von Balthasar, Credo
With the explicit disclaimer that these are simply my thoughts, and arguably quite speculative ones, though intended to be rooted within the received and historic Christian teaching:
My answer would be "love", but not that God needed an extradivine world in order to love, but instead the creation of the universe is itself God's own loving made manifest--God is Creator of the heavens and the earth, neither by necessity nor by happenstance; but because He is Who He is. There can exist no subsequent "idea" in God, there was never a "time" when God suddenly decided to create as though it were an idea beside Him that suddenly then occurred to Him that He should do it; thus the creative work must be understood as in some sense intrinsic to God. We do, after all read that He has chosen us in Christ before the foundation of the world, "he chose us in Christ before the foundation of the world to be holy and blameless before him in love. He destined us for adoption as his children through Jesus Christ, according to the good pleasure of his will," (Ephesians 1:4-5). The creative act is an expression of love, of the very love the Father has for His Son, and the Son has for His Father; and in this love we are not excluded, but included; in the Father's own love for His Son He loves all who are in Him, and in the Son's love for His Father He brings with Himself all who are in Himself before the Father. There is therefore a permanence and completeness--a perfectness--of creation that springs from God's own interior, kenotic love, one that includes both the beginning and the end of all things.
-CryptoLutheran