Spoilers ahead.
This is inspired by 'Crisis on Two Earths', a Justice League cartoon, in which the villain pointed out that each decision made was made differently in an alternate earth, thus creating alternate earths, and thus making free will of no effect because for every world where you chose good, in another world, you chose evil.
So, he searched for the original Earth to blow it up and destroy all reality.
Now somewhat accepting that postulate, and somewhat twisting, and somewhat advancing it, we come to the Little Multiverse, the Many-Faced Multiverse.
The MFM is seemingly one of the larger Little Multiverses. It has millions of alternate realities in it, or so theory goes. All scientists that are aware of alternate realities believe it to be The Multiverse.
Whenever a decision point of some significance is reached by a sentient being, it results in the splitting of the realities into A and B. No more. No less.
One would think this leads to the Doom described in 'Crisis', but not so.
For one, the decision is frequently not between good and evil, but between evil and evil.
"Shall I, a mugger, rob and beat this man, or shall I rob and murder this man, my victim?"
Two realities go forth from this moment.
There are various entities, some who hate each other, of great power. Some call them gods, others personified natural forces, others enlightened ones, but little is known of them as they are very hard to understand, and at best they speak in riddles. Also meeting them is hazardous as they seem to in the words of one materialist scientist 'carry a personality destabilizing field which is intrinsic to their nature', or as a priest of the True Knight declares 'the glory of their presence is too great for mortal mind to bear'.
They seem to travel freely from alternate reality to alternate reality. They seem to be able to die, but with difficulty. They also seem to be able to be wounded and recover, although this may take centuries. They are effectively immortal unless killed, and killing them takes some serious doing, or a very, very effective ambush with a nuclear missile (this is the equivalent of shooting Billy the Kid in the head with a .22 while he sleeps.) Actually killing a fully aware Traveller might take a supernova.
The True Knight, the Market Lady, Knife, Gravedigger (out and out evil), and Warlord are but a few of the approximately 2-3 dozen Travellers that seem to be currently operating. Another 2 dozen or so are severely injured or retired. An unknown number are dead or otherwise transformed.
In this MFM, dreaming is enhanced for indigs, although visitors, such as versers receive no such enhancement. Very vivid dreams are common for indigs, often enough dreams that continue on night by night with the same story advancing forward for some time, even months. Cases exist of people who have had the same story dream go on for decades.
This creates a strong culture of art and scientific invention with the dreams serving as inspiration.
Government forms vary widely from royalist monarchies with purple and gold, to totalitarian dictatorships in blood and concrete, to democracies with MacBurger drive-ins to treehouse villages and everything else.
Typical world-jumping devices, (which does not happen in every world, at best, one in ten), will arrive at a distinctly different world in some degree as the fine gradations to jump to a world very close to one's own are not possible in the beginner level dimension jumpers.
Now theory suggests that billions or at least millions of worlds exist. What current theory does not take into account is that worlds that are deeply parrallel, sometimes in ways that don't seem immediately apparent to the eye (but are parallel on a moral level) tend to merge back together. You can have a world of skyscrapers and a world of treehouses merge, while a world which is nearly totally alike, but in one the robber beat and in the other the robber murdered will not merge.
It is possible to have more good or more evilly inclined worlds than other. It is the number of decisions made that matters. And worlds that go wholly or even very strongly evil tend to extinguish themselves, thus removing them from the total number of decisions made.
It is possible to find the Prime world, and once there, the visitor, if an indig, realizes a shocking truth. He becomes a Traveller. He finds that each 'him', each doppleganger of himself in another reality is but another face for him. He exists across possibly thousands (usually a few hundred) timelines. And all this mentality crashes in on the indig who visits the Prime world. All of his faces compete to rule in the new mindspace.
For many, this kills them. For others, it leaves them catatonic (indeed most pass through a stage like this for at least a few days).
The balance of good and evil choices made, the strength each mind has, the number of minds involved in this internal struggle makes this a vast conflict inside the newborn Traveller. Typically a Traveller has to partially de-phase from reality in order to 'get enough room' in his head. This contributes to making them seem inhuman.
This is why dreams seem so vivid. Its because the dreamer is acccessing the life of another part of himself.
And it does happen that moral choices matter, the MFM as a whole can be tilted toward good or evil by the choices each person makes.
Its also true that if you kill your dop, you gain him as an annoying voice in the back of your head. So, yes, you can become a Traveller by killing all your other dops.
Some people have relatively few dops, and these make for weaker and saner or more human Travellers.
So perhaps this is not really a multiverse at all.