Obviously I'm a little sceptical about "modal realism". Lewis claims it's "simpler" in an Occam's Razor sense, in that it allows fewer CLASSES of objects, by not making a hard-to-determine distinction between "actual" and "unreal", claiming actuality is indexical, within the "world" you inhabit, not across all "worlds". Thus, you have a hugely expanded number of entities, but fewer classes of entities.
That's somewhat similar to the way "many-worlds" multiples the number of entities a billionfold every second, but claims to be "simpler" in an Occam's Razor sense because it doesn't require a complicated "collapse" of possibilities.
I'm a little sceptical that a quadrillionfold increase in entities is worth the loss of one or two complicating factors, eliminating an extra step or two. But I'll discuss Occam's Razor soon.
Lewis' PLURALITY OF WORLDS discusses this in depth. I haven't had a chance to read Max Tegmark's OUR MATHEMATICAL UNIVERSE, in which he espouses his "mathematical universe/computable universe" theories, but I have read his earlier "The Mathematical Universe" paper. To be fair, he did touch on Godel's incompleteness theorem and the implications for his computable universes, but since Godel's theorem applies to all but the simplest of mathematical systems, I have trouble swallowing it in this case.
If Lewis is right about causally independent "worlds" with no connnection to each other, there is no way to have one god who created it all, since that Creator would have no connection to all existences. (Which is odd, since both Lewis and Tegmark seem to agree the world of mathematics and sets are "real"---yet mathematics and sets seems to apply across all such "worlds".)
However, it seems you would have to allow all sorts of worlds that might have "gods" ruling them---worlds that have the Olympian gods, the Norse gods, the Mayan gods, worlds that reflect the Divine Comedy or a strict fundamentalist's world that began in 4004 B.C. For a nontheist sceptic, the result would be a cosmos where there could be no one overriding Creator--but it's a pyrrhic victory, because there seems to be no way to eliminate multiple Jehovahs, Zeuses, Odins and every other god ever conceived...not once, but multiple times.
Oh, here's a paper by Stoeger, Ellis and Kirchner on "Multiverses and Cosmology: Philosophical Issues" that addresses many of these issues.
Next time: ENSEMBLES: AN OVERVIEW
Most scientific theories have quantum-mechanical worlds. But modal realism is philosophy, which doesn't need scientific justification or proof. Even so, a world with Harry Potter or even Zeus would probably have most of the same laws as ours---just with a few more added, or a few exceptions added.
A delightful series of stories hinging on the three "laws" of magic are the Harold Shea stories by L. Sprague de Camp and Fletcher Pratt.
BTW, I would love for some parapsychology department to "test" the laws of magic. To my knowledge no one has rigorously tested them. Wouldn't it be funny if they actually work? The Law of Contagion is oddly like quantum entanglement...