Reality Is A Novel.
I'd like to propose a modified Many Universes theory. Rather than saying every possible universe exists, I'd say, rather, that there is a sequence of possible universes, akin to the drafts of a novel.
We're living in a draft version of the universe—and there is no final version. The revisions never stop.
From time to time it's possible to be aware of this. In particular, when you relax and stop naming things and forming opinions, your consciousness spreads out across several drafts of the universe. Things don't need to be particularly one way or the other until you pin them down.
Each draft, each spacetime, each sheet of reality is itself rigorously deterministic; there really is no underlying randomness in the world. Instead we have a great web of synchronistic entanglements, with causes and effects flowing forward and backwards through time. The start of a novel matches its ending; the past matches the future. Changing one thing changes everything. If we fully know everything about the Now moment, we know the entire past and future.
With this in mind, explaining an given draft of the universe becomes a matter of explaining the contents of a single Now moment of that draft. This in turn means that we can view the evolution of the successive drafts as an evolution of different versions of a particular Now moment. As Scarlett's climactic scene with Rhett is repeatedly rewritten, all the rest of Gone With The Wind changes to match.
And this evolution, too, can be deterministic. We can figure we think of there as being two distinct deterministic rules, a Physics Rule and a Metaphysics Rule. The Physics Rule consists of time-reversible laws that grow the Now moment upwards and downwards to fill out the entire past and future of spacetime. And we invoke the Metaphysics Rule to account for the contents of the Now moment. The Metaphysics Rule is deterministic but not reversible; it grows sideways across a dimension that we might call paratime, turning some simple seed into the space-filling pattern found in the Now.
The Metaphysics rule is...what? One possibility is that it's something quite simple, perhaps as simple as an eight-bit cellular automaton rule generating complex-looking patterns out of pure computation. Or perhaps the Metaphysics rule is like the mind of an author creating a novel, searching out the best word to write next, somehow peering into alternate realities. Or, yet again, the big Metaphysics rule in the sky could be the One cosmic mind, the Big Aha, the eternal secret, living in the spaces between your thoughts.