Previously I discussed teams and organizations, people and places on Earth, and space and other dimensions. Now we get to one of the most iconic parts of cape comic universe: Parallel universes!
The idea of parallel Earths with alternate versions of heroes seems to have debuted in "Wonder Woman's Invisible Twin" from Wonder Woman #59 (May 1953) but it wasn't fully expounded upon until the classic "The Flash of Two Worlds" from Flash #123 (September 1961) with the concept of "parallel Earths occupying the same space but vibrating a different frequencies". Substitute the Earths with Universe and you've got a Multiverse. Crisis on Infinite Earths amended this even further by making the Multiverse a set of INFINITE parallel universe occupying the same space and vibrating at the same frequency, right before destroying all of them. Oh well.
There's a big difference in the way Marvel and DC approach multiverses, with Marvel favoring Divergent History and DC favoring Parallel Evolution. Divergent History is just that, it starts with an established real-life or comic book history and asks What If...? So we get a world where Captain America didn't wake until 1984 (What If Captain America Were Revived Today?) or where Sue Storm abandoned the Fantastic Four but Spider-man became a permanent member (What If Spider-man Joined The Fantastic Four?). The Divergent History method is a way to revisit old concepts and roads not taken. The Parallel Evolution method tends to change the setting and rework concepts and archetypes to conform to that setting. For example, a world where Kal-L of Krypton landed in USSR in the 1940s (Superman: Red Son) or one where Bruce Wayne is bound to the Demon Etrigan and lives in a steampunkish version of Camelot (Batman/Demon: A Tragedy). The Parallel Evolution method is a way to take a fresh look at concepts and inject a little weirdness into a multiverse.
You don't have to choose between the two methods and it's quite possible to mix and match between the two approaches (90s What If...? and some of the later Elseworlds did this).
The exact difference between a Multiverse and a Timestream is hard to decipher. Indeed, Crisis on Infinite Earths featured heroes going to the wild west, WWII, and the medieval era to destroy the Anti-monitor's evil tuning forks, seemingly conflating both Temporal and Universal travel. Marvel tends to treat pasts and futures as simply other worlds in the Multiverse; indeed, for a long time the rule was that an attempt to alter the past by time travel resulted in the creation of a new Universe while leaving the traveler's home Universe unaltered (This rule is being broken/bent in modern Marvel comics). After the destruction of the Multiverse, a few DC writers (Mark Waid is the one that most easily comes to my mind) proposed a time-equivalent of the Mulitverse called Hypertime. To paraphrase the above line about the Multiverse, Hypertime could be interpreted as "infinite parallel timelines occupying the same time but vibrating at different frequencies." So while the Multiverse and Hypertime are definitely not the same thing, they're close enough to be considered the same thing by most people.
There's a problem that I've been dancing around and that is the "infinite" part of the Mulitverse/Hypertime equation; namely, it makes all the heroes' and villains' actions meaningless. For every life saved in this world, there's another were one or more were lost. If all actions are a finite drop in an infinite bucket then it can quickly seem pointless from a character perspective. A good examination of this comes from Owlman in the animated DC feature Crisis on Two Earths (although Batman is very badly written in it). A way to combat this to make the Multiverse/Hypertime at least appear finite in a way similar to the "snowflake Multiverse" of Warren Ellis' Planetary or DC's post-Infinite Crisis Multiverse. So how do you present a somewhat satisfactory answer as to why the Multiverse/Hypertime is limited? I'd personally take a page from atomic structures: The strong nuclear force can only hold together a nucleus that has about 100 protons in it; go above 100 and it will start breaking apart via radioactive decay. But there are atoms with over 100 protons, the just violently disintegrate in very short times. So certain frequencies in the Multiverse/Hypertime are inherently stable and past a certain frequency there can be no stable Universes/Timelines.
Confused yet? It really confuses me too, although I still love it. I've probably rambled enough so let's get to the details already, yeesh. I use Universe and Timeline interchangeably below, so don't be afraid to make an alternate present where everyone's evil or an alternate Earth that's a sci-fi future.
The Evil UniverseA mainstay of fiction sometimes called a "Mirror Universe", this reality features evil counterparts of heroes and good counterparts of villains. In some cases, history may be altered or reversed, such as making England a colony of America that revolted and gained independence in 1776. For very brief adventures, just switching the moral leanings of your main reality's supers while keeping a realistic population should do well enough to convey differences; for extended looks at this type of universe, shades of gray can make the reality more engaging. For example, the counterparts of villains might be "good" but are still brutal or the counterparts of heroes are "evil" but have a certain code of honor.
Examples in comics: DC's Earth-3, the "shadow Earth" created by Warlock/Magus in Marvel's Infinity books
The Old UniverseIn this reality, all major heroes and villains debuted at a specific time decades ago, aged, and have children. This creates a much longer, stronger Superhuman culture in this reality. It's also not unreasonable to expect more teams than usual here to accommodate the much greater number of Supers. Although this type of universe shows up in comics, it's a bit harder to adapt to a roleplaying game.
Examples in comics: DC's Earth-2, some issues of What If...?
The Nazi UniverseA perennial favorite of "speculative fiction" is the alternate time/place wherein Nazis won WW2. In comics it generally manifests as turning most post-1940 supers into Nazis. Depending on the current year of this reality, there can be a lot of fundamental differences between this Earth and the "main" Earth of the setting.
Examples: DC's Earth-10, Marvel's Hauptmann Englande and Thunder Guard universes.
The Near Future Dark Age40-100 years in the future, The Team is dead and so are most heroes. Megacorps/Terrorist organizations/Criminal syndicates rule the world (or maybe just the USA). The government is too weak or corrupt (or both) to put any pressure on the corporate juggernauts. The poverty gap is wider than ever. Cyberpunk elements become more pronounced. Heroes are more pragmatic in their methods and most villains are only in it for the money.
Examples in comics: Marvel's 2020 and 2099, DC's Booster Gold and Zoom futures
The ApocalypseOverlapping with the above, the Apocalypse reality is about 50-200 years in the future after some great disaster has ravaged the world. The great disaster can be any number of things, including but not limited to: Nuclear war; biochemical war; magic gone awry; alien invasions; science gone awry; demon invasions; zombie uprisings; robot uprisings; or pretty much anything else. Humanity is perilously close to extinction and very few supers are around in this reality.
Examples in comics: Marvel Zombies, DC's Kamandi
The Cosmic Future500-1000 years from now, Earth and Humanity are part of the cosmic stage, colonizing and exploring the fringes of space. Aliens that had previously been enemies of Earth may have become allies or vice versa. In some cases, Aliens that hated each other have teamed up to fight Earth! The equivalent to The Team in this era is heavily inspired by the ancient legends from the Super or Heroic Era of Earth and tend to be almost dangerously idealistic.
Examples: Marvel's original Guardians of the Galaxy, DC's multiple incarnations of the Legion of Super Heroes
The Missing Hero UniverseIn this reality, the most popular "big name" hero of the main reality (usually equivalent in influence to Superman or Captain America) (re-)appeared significantly later or didn't appear at all. Without this hero's influence the Super society is a bad state and the general public is fearful of supers. If the hero ever does appear he or she might cause some major shake-ups in society and culture.
Examples in Comics: Marvel's various What if Captain America was revived today? stories, DC's JLA: Nail
The Reverse-Sex UniverseLike the Nazi Universe, the Reverse-Sex Universe is a common cliche but it has even less world-building in place. In this reality, all Supers (or all beings) are the opposite sex...men become women, women become men, and I guess sexless entities become hermaphodites and vice versa (dare you enter that magical realm?). If you use one of these realities then please take the time to do a little world-building so that the world isn't just "the same as the regular one but Superman has boobs and a vagina!!!"
Examples in comics: Marvel probably has one that I can't find, DC's Earth-11 and a Pre-Crisis universe.