There are other kinds of multiverse theory that are less speculative. For instance, if “inflation” is right then it seems likely that it produces “eternal inflation”, which yields infinitely many universes that might have different fundamental constants (though the same kind of fundamental laws).
(And of course either MWI or a spatially infinite universe will produce what might as well be a multiverse, if you don’t need the fundamental constants of physics to be different between “universes”.)
Speculation.
There are other kinds of multiverse theory that are less speculative. For instance, if “inflation” is right then it seems likely that it produces “eternal inflation”, which yields infinitely many universes that might have different fundamental constants (though the same kind of fundamental laws).
(And of course either MWI or a spatially infinite universe will produce what might as well be a multiverse, if you don’t need the fundamental constants of physics to be different between “universes”.)