@ben d sorry for the loooong delay on this.
It's not only possible that there are other universes (if we understand "universe" not to mean "everything that exists" but rather "the largest scale structure the visible universe is connected to"), but probable if inflationary cosmology is correct. Inflation is well-evidenced and answers a lot of sticky problems like "why are inhomogeneities (e.g., huge patches with the same anomalous temperature compared to the mean) in the CMB so far apart that they couldn't possibly be causally related" and "where are the magnetic monopoles" and "why is the universe so flat."
If inflation is true, then what we call the visible universe is just one bubble where inflation decayed: inflation would be ongoing elsewhere, quickly dominating, and spontaneously decay into other bubbles which would be other universes.
For the next question, I have to nitpick "if it came into existence from nothing:" that isn't suggested by Big Bang cosmology. The absence of time and the universe that we see is not the same thing as "nothing," for instance as far as we know there could have still been fields (such as in the eternal inflation scenario), and things like that could have existed forever as far as we know. Big Bang is "creation ex nihilo" agnostic in other words.
However we can speculate about whether there will be a time in the future where pre-BB conditions might be reached again: the answer is "boy is this question hard to answer in English," and by that I mean I have to use terms like "future" where time may have no meaning and "here" where space may have no meaning, but I shall do so colloquially anyway.
Yes, it's possible that the future of the universe here (our bubble) might re-attain pre-BB conditions according to Penrose, but I wouldn't say that this has a scientific consensus behind it. His idea is essentially that in the very distant future, when a bunch of speculative stuff has happened like protons being unstable and have decayed, after a hypothetical period where all that exists when the last black holes have decayed would be a world of positronium, that this positronium too would eventually meet its end with self-annihilation, leaving only photons (or more technically, only particles with no rest mass).
For relativistic reasons, Penrose argues that at this point, space and time lose all meaning, and very distant photons will be colocated because the space between them would have no meaning, and likewise with time (which is defined by the entropic gradient). Now, I'm not on Roger Penrose's level (I wish, I am but a grad student), so I'll just repeat that there's definitely not a concensus on his idea; though the man himself is a serious and respected theorist. So, we will have to see whether anyone else runs with such ideas.