God's Big Project is Making Heaven (which is mostly Populating Heaven).
But why did he do this?
Well, what is the Christian Heaven? It is a family of God's children.
Why would God want a family? Well, why does anybody want a family - because a good family is an environment of love. So first of all, God wants to live in an environment of love.
God does not need this environment in order to survive, but wants this in order to be happier. From God's point of view Heaven is a place for joy, not a place for survival. I infer from this, that for us too, Heaven is meant for our joy - and not, therefore, as our only hope of survival.
(To be clearer - I regard it as an error to conflate the happiness of Heaven - for those who want it; with the idea that Heaven is our only possibility of eternal life. 'Mere' eternal life can be had otherwise than Heaven.)
I assume that when Jesus talked of his gift to us of Life Everlasting, in the Fourth Gospel, he meant to imply much more than mere personal survival after death - because the bulk of the Fourth Gospel is taken up by poetic/ symbolic descriptions of the qualitatively-enhanced nature of this post-mortal and Heavenly life.
So, firstly, Heaven is made as a better environment for God; better than the alternative of (more or less) relative solitude. And then it is made as a better eternal, post-mortal environment for Men.
(But not for all Men; only for those Men who want it, who freely and consciously choose it.)
Having made a place, and decided it is to be like a family; what then do people actually Do in Heaven? What could possibly be a sufficient occupation for eternity?
Only, I suggest, a job that is both open-ended and intrinsically gratifying - a job that never gets finished, but which is satisfying all the way and at every stage.
The only such work is creation, in its many and various modes. And for creation to be lastingly satisfying, the 'products', the outcome-of-creativity must be eternal.
So if you and I are to be creating forever, this means we must participate in the divine work of creation - because only that is eternal. We must become co-creators with God: we must become gods alongside God.
To clarify, God is the primary creator, we God's children. The plan is that some, at least, of us will rise to become 'secondary creators' within the primary creation; but this secondary creation becomes itself a part of all creation. God's creation is ongoing and increasing all the time; and our personal creative contributions to this are woven-into the eternal fabric.
This means that, as well as being a family, Heaven could also be called a 'collegial' situation in which some (at least) of God's children have themselves fully grown-up to become immortal (resurrected) gods. Heaven requires gods as potential colleagues, as full collaborators, in the everlasting, ongoing work of creation.
In sum, the making of Heaven is the making of a situation in which everlasting life can be a permanently fulfilling activity; a place, therefore, of love and creation.
Note: These musings are derived from the early parts of William Arkle's booklet Equations of Being.