@Zaku
To be exact, @Jeruba is pointing out that universe counts for everything that exists anywhere. With that definition, no new universe can come to be, since it would automatically be a part of all that is.
For the sake of discussion, I think it’s ok to posit a “universe” separate from ours in that it does not interact with our universe in any manner.
That is, we have a timeline, and they have a different timeline, and maybe they have a different physics.
Now, since they are still conceptually part of all that is, they are not a separate universe, but they are completely outside our reality bubble, and for all intents and purposes are close to the definition of a separate universe.
Or, one might sensibly just limit the definition of universe to the observable reality we live in, and the parts of it that seem to act as a coherent whole, rather than the bigger net of all being.