Concepts and Places
Big Three: Time Travel, The Future, and Death
Time is not a thing. It isn't a substance, it isn't a place, it's just an idea. A measurement. How we keep track of events, and it only goes one way: forward, at one hour per hour. Trying to move through time any other way is an exercise in futility. No being, no matter their abilities, can truly affect time, much less travel backward through it - it's just a series of events, and what has happened, has happened. A person's perceptions and body can be accelerated or slowed, but effects that appear to stop time are actually only doing this, accelerating the individual. Some abilities may be able to observe the past, but never affect it.
Just like time is a measurement, the future is just an idea, a way of explaining plans or expectations. The future doesn't exist because it hasn't happened yet, and nobody can 'see into it' or be certain what it may hold. Fate and destiny are inherently incorrect and indeed impossible concepts, and the best one can do to predict the future is analyze a huge data set with a good understanding of how people and the world work and act.
Death and Afterlife
What happens at the point of death is for the most part fairly clear - the body shuts down, and the soul seems to quickly fade away until it winks out. Nothing 'leaves' the body when it dies, and nothing 'survives' death unless someone can very, very quickly move the soul elsewhere. Reviving someone will make the soul blaze back to life as well, usually without any loss or damage unless the body suffered brain damage while dead. What happens after death, on the other hand, is a total unknown - and it's impossible for it to be any other way. Nothing exists that can prove or disprove what may or may not exist postmortem. Barring specific powers that cause it, we know that 'reincarnation' doesn't happen, but anything beyond that is a mystery.
Other Concepts and Places
Alternate Realities / Other Earths
At least among those who have had a modicum of interaction with the anomalous, it is well known that ours is not the only existence. Other realities and timelines similar to our own but distinct in their own ways exist, and can even occasionally be touched and reached into - though crossing the gap and actually managing to step into another reality is risky at best, since so little is known about the possible effects of interdimensional travel. The more different a reality is from ours, the more difficult it is to reach, and the less precise any attempts will be, possibly getting one similar to the target reality but not quite the right one.
Officially, it is unknown whether life on other planets may exist. Even if life exists, even if it develops full sapience, even if it somehow does so at such a rate and from such a starting point as to be a sapient species at the same time as we ourselves exist, and even if that species used something we could recognize as communication, managed to journey into space, and - against all odds - somehow left their own solar system and reached another… the chances are infinitesimal that they would ever eventually find and reach our own system and planet, and by some coincidence it happened to be any time near the present. A very, very generous estimate could speculate that there may be perhaps half a dozen other worlds with sapient species in our galaxy, with merely half of them being both industrialized and non-extinct, perhaps two having left their planet, and perhaps only one having entertained the idea of deep space exploration - yet even they probably wouldn't have yet managed to find some form of faster than light travel, or had the time to send a cryostasis ship at near-lightspeed into the void, much less toward Earth.
"Empty, but with infinite possibility." Half realm, half thin, vague field of psychic energy from the extremely low level telepathic broadcasts of humanity, the Dreamscape is shaped and influenced by the thoughts and desires of those who interact with it. In its natural state, it's blank and empty, but it's never seen in is natural state - for the moment it's observed, the will of the observer begins to affect it. It is from here that the energy needed to create thoughtforms comes, directed by focus and conviction. Note that the field doesn't react enough to any single will to enhance telepathic ability at all, nor to create a thoughtform without a direct link to the realm. Gathering enough total focus, desire, energy and will to direct it enough to form something real is, by necessity, a communal activity.
Not every 'dream' comes to fruition, and not every idea is remembered. As long as someone lives who remembers an idea, or the existence of an object, it is safe. However, once something is lost to the world, with not a single soul keeping it in memory, a shadow of it forms here, like a leak from the Dreamscape mourning what once was or could have been - or an unrealized dream. By its very nature, the Forgotten is very difficult to access, for if someone were to comprehend something here, it would fade until once again forgotten. If something could be pulled out of this realm, and not be known until it was safely away though…
A full realm of its own, with unique geography and life, Faerie is continuously shifting, coming into contact with the mortal world at different points. These rifts connect to places based not on relative location, but on ideas and associations, and can last anywhere from a couple minutes to a few hours. Faerie is saturated with magic, which affects the life that's developed there and sometimes that which stays there too long - the unique properties of the place, combined with this, can result in dramatic changes to humans in particular if they live there from a very young age.
Faerie is, naturally, home of the Fae, with multiple small communities and individual domains. Its shifting nature lends itself well as a shortcut for travelers who understand the realm, since a short distance in Faerie may bring one to a place with a very different association, with any brushes against the mortal world coming out far from where the traveler originated.
The manifestation of a being's uniqueness, self, and personality. It 'grows' with life experience, and small portions can be mostly-safely poured into things that have special meaning to them - it regenerates slowly if someone 'puts their heart and soul' into a project. Intrinsic to an individual, and fades into nothing if they die without it somehow being moved elsewhere - but can return if the person does. All unique entities with a sense of self and a personality have a soul - from humans, to sapient synthetic beings, to living, feeling thoughtforms.
The Spirit Plane
The plane of spirits is not a realm or world of its own, but a layer of reality existing in the same space as the material world and mirroring it. The landscape is like a shadow cast by the physical, reflecting changes made in it and making anything non-organic visible, but lacking anything from there that is or once was alive or part of something living. It is home to spirits of all kinds, though any being in it unhindered by a physical body can move by thought alone, in any direction and don't interact with real matter - the buildings and ground are not actually part of the plane, but only visible from it. This means that the landscape itself can't be changed, or even really interacted with besides just acting and pretending; the reflection is one way.