The Soul Wanderers

In this post, I'll look at 3 more possibilities for other worlds. The first 3 were in the previous post that are worth reading first so you know what on Earth is going on (actually, its not on Earth at all).
So here are three more possibilities for other worlds:

4) Parallel Worlds from Time Travel
Time travel works differently depending on your theory of time. By theory of time, I mean how time "works". Regardless of how you do it (via relativity, a wormhole, a TARDIS, etc.), what is more fundamental is the nature of time itself. Some features of time could be that there is or is not an instant in time that you can call the present, that time had a beginning/end or does not, that you can only change things in the past that will not cause too much of a paradox, that all time travel is determined, that time itself is or is not determined...and so on.
What concerns us here is whether parallel worlds will be created because of time travel. Imagine if you could create a world just by travelling through time! (Ok, perhaps it isn't fair to say "just" travelling through time, unless you're the Doctor). In the scenario that the past is determined but the future is not, then if someone travels to the past, then we have a problem. Everything is set in our determined past except for the time traveler going there, because from the point of view of the time in the past, they are in the future, which is not yet set. One possibility is that this creates a parallel world, one that has the same past as the "regular" world until the point in time that the time traveler goes back to, and at this point, time expels him into a parallel world to keep the regular timeline intact, like so:

The parallel universe will start out very similar to the regular one, but it will diverge to become more and more different due to the effects of the time traveler. So after some time, they could be completely different worlds, with different people and histories, different societies and technologies.
If people time travel enough, there would be many parallel worlds, some similar, some vastly different, and the parallel worlds could spurt parallel worlds which could spurt more parallel worlds so that you could get a mess like this:

Here, the bottom line is the regular timeline which fractures to many different worlds because of those clever time travelers. So time becomes even more relative than one would suppose just with the special theory of relativity: you can't even say that these worlds have the same time at the start since the parallel world is created in the past of one timeline (the original one) and the present of another (the offshoot). That's worth thinking about for a moment.

5) Quantum Mechanics (QM)
In QM, what can happen, does. At the subatomic scale, things don't happen in the relatively predictable manner that they do in the world we are used to. Things can be in more than one place at the same time, particles at opposite ends of the universe can be tied together faster than the speed of light, and it is impossible to accurately know both the position and momentum of a particle.
One interpretation of how QM works is the Many Worlds theory. In this scenario, each quantum mechanical possibility occurs in a different world. This is in contrast with the more common Copenhagen Interpretation. In the Copenhagen Interpretation, particles exist in many states at once, but when they are observed (by a person or equipment that takes a measurement) the many states collapse into one, known as the "collapse" of the wavefunction. However, another interpretation is the Many Worlds theory introduced by Hugh Everett, in which every quantum mechanical possibility happens, but each occurs in a different world. So everything happens, and there are nearly infinite possibilities! So think about every possible universe, those that are only a little different to ours (e.g., one in which your favourite colour is red instead of purple) and ones that have entirely different planets, solar systems, and galaxies.
To tell you the truth, when people say that there infinite number of "you"s in parallel universes, the very first thing I think of is: "So there is a world where I am an elf queen in Middle Earth with magic powers?" Um...yes. I guess. If that magic follows the physical laws (see 6) below for universes with other physical laws). But whether it is really "me" is another matter entirely. If you believe that you have a soul, or anything beyond the physical matter that makes up your body, then you'd have to say that unless your soul is also duplicated (something QM doesn't tell us about), then it is not you, but just a being that has a similar body to you.
Another aspect of many worlds in quantum mechanics are the creations of universes. This can be seen analogously to virtual particles. Virtual particles pop in and out of existence in the vacuum of space, being created and, most often, destroyed by annihilating with their antiparticle (for example, an anti-election is a positron, which is identical to an electron except for the fact that it has a positive charge).
Likewise with universes, embryonic universes pop into existence in the vacuum, and although most just get "swallowed up" again, some, like ours, grow to become an actual universe. It seems to me that it's arrogant to assume that our universe was the only special one to develop into a universe: why shouldn't others do the same? Here are our other worlds, and although they may be entirely separate "bubbles", it may be possible with advanced technology or magic to access them.

6) String Theory
String theory is an elegant attempt of theoretical physicists to find a theory of everything, the goal being to unite quantum mechanics (which describes microscopic objects) and general relativity (which describes macroscopic objects). The basic idea of string theory is that at the most fundamental level, matter is made of strings that vibrate. These are one-dimensional strings, and their properties are determined by their vibrations. There are many different versions of string theory, but they all involves extra dimensions, 10, in fact (though with M-theory, a kind of string theory that unites many of the different ones with 10 dimensions, has 11). They often say that the other 7 dimensions are "curled up" so that we don't perceive them and seem to be living in a 3-D world (not including time dimensions).
However, it is also possible that these higher dimensions are larger, and that we are living in a higher dimensional universe (like those mentioned under "Other dimensions" in Part 1). If that's the case, then we can be living in a higher dimensional multiverse that may have higher worlds that encompass more dimensions, though it is also possible that there are other worlds like ours of the same dimension that are "branes" in the "bulk" of the multiverse (I didn't make those terms up--check if you don't believe me!).
So we may be a 3-D brane hovering within the 11-D bulk, and there may be other branes of other dimensions, say, a 5-D brane or a 7-D one, that are also hovering in the bulk. These branes can collide, merge, and other universes can be created by the collision or fission of branes.
These worlds can have different physical laws, and because of some having higher dimensions, things would be very different to our 3-dimensional world. If we traveled to them, we would perceive them all as 3-D worlds because we can't sense higher dimensions directly, but there are other ways in which we could detect them scientifically (e.g. by the "imprints" of higher dimensional objects talked about in Part 1).