This is really just me brainstorming some various ideas that have popped into my head over the last couple months. Nothing really fleshed out and who knows if they'll develop into anything, but figured I'd share the basic ideas.
1) Gatewalkers: The Endless Worlds. This idea is one of the better fleshed out in my head ones but harder to describe. Essentially, it's 2070 and humanity has proven beyond any rational doubt two things. The first is that it is impossible to move faster than light by any means unless a power source is developed greater than that of the Sun with a size no greater than a breadbox. The second is that other realities do exist and, with current power, gateways could be opened to alternate Earths. Since global population accelerated to the point where Earth is barely sustaining it's 14 billion inhabitants, and the technology needed for enough space stations to matter is still decades away, the decision is made to begin building these gates and find uninhabited Earths to begin populating. At first it goes remarkably well - the first six Earths we find are remarkably similar to our own (only variation being different plants/animals or different placement of landmasses - or both) with the notable exception being that no sapient life ever evolved on them. With no need to terraform, colonization began rapidly. However, laws were passed to limit how many could migrate - no more than 20 million per every 400 square miles, and some heavy restrictions on what they could and could not do - and could and could not bring with them.
It wasn't until the 7th gate that we ran into problems. See, we weren't the first Earth to discover gate technology, and attracted the attention of another one when we opened a gate in the middle of a populated city - populated by human-level intelligent theropods that had about a million years of evolution technologically and 65 million years of evolution biologically on us, and they weren't happy that humanity was starting to snatch up the uninhabited worlds.
By now, dozens of worlds have been uncovered and multiple civilizations, some ranging from alternate versions of humans where society developed different (and one world where humanity developed differently) while others are sapient creatures that developed in the absence of humanity filling that niche. That plus the discovery of worlds where the natural predators make T-Rex look like a rabbit forced the assembly of what the players would be - PORTAL teams (Preparatory Operation and Reconnaissance Tactics and Logistics) that are the first into any new worlds, given the best of human technology and genetic engineering, trained to discern if a world has a sapient race, asses risk of natural predators, assess liability, make peaceful contact with local sapients if at all possible, etc.
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The Desiccated Seas - In this setting, water is the most precious commodity, making the conflict over oil we have today look like children fighting over blocks. A disaster struck, one of divine retribution - and the oceans were turned to seas of dust. Now sentient races must struggle to survive off the few remaining lakes, rivers, and oases while horrific creatures evolve in the wastes between, feeding off the magical energy left behind from the Desiccation. Waste Walkers are the heroes of the setting, people willing to travel between the various humanoid settlements to secure agreements to keep people alive - but eveyone knows the clock is ticking on how much longer the water can last.
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The Thousand Realms - Seems like I'm into the reality hopping stuff, because the Thousand Realms is another setting in that vein. A fantasy setting, the Thousand Realms takes place in a universe where each "material plane" (aka, each Realm) is what d20 refers to as a demiplane - varying in width from tiny (two or three miles) to massive (2000-3000 miles, a joke to us who are used to light years. What makes the Realms unique is that physics is a joke - while many realms use gravity the way we do here, others are twisted mazes of caverns where down is whatever direction is beneath your feet, and others are floating, spherical islands where flight is just a minor effort of will.
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The Clockwork Jungles - Combining my love of the post apocalyptic and steampunk, plus a recent re-watching of the Matrix, in The Clockwork Jungles steampunk machines were imbued with a degree of intelligence that went beyond simple AI and they turned upon their magic-wielding masters - and won. Humanity and other sentient creatures are either used by the machines as a form of food source, drawing the arcane power inherent in all sentient creatures to continue there expansion and advance. Humanoids were forced either underground or into pocket realities where they wage war against their mechanical overlords in a desperate attempt to reclaim their home for themselves.
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The Shards of Eldron - A sci-fi fantasy in the same way Star Wars is Sci-Fi, the Shards of Eldron takes place on inhabitable meteors orbiting a gas giant. The people of these meteors (called shards) wage war against each other using advanced technology, combat prepared machines, and a mystical energy that can be channeled through certain individuals. Other shards work together, form alliances, even full out empires and confederations, allowing for games that involve combat, intrigue, or any combination thereof.
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Innoculation - Lovecraftian horrors from beyond time and space are beginning to slip back into our reality, that of Earth. In response, Earth's defense system is activated, one that's been present for millions of years before humanity - and people become endowed with strange powers while life develops into new, unexpected forms, behemoth and powerful animals that can pose a threat to these beings. Unfortunately the truth of what happened is known to only few, and humanity is busy using these new tools to war amongst themselves instead of uniting against foes that could consume reality.
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The Cracks in the Sidewalk - A simple premise for a simple setting - there is a world that exist hidden from the one we know, one where darkness grows and festers and waits for the right moment to strike. These are the things that go bump in the night, the boogymen and banshees and vampires and hellspawn that would love nothing more than to twist the world in their image and reestablish their former dominion in the hearts of mankind. All that stand between humanity and the horrors of the cracks are a long standing, forgotten brotherhood that seeks nothing more than to preserve society form that which lies beyond it.
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Splicers - Another simple setting - there are men and women who can warp time itself though sheer willpower, organizing into secret societies with various agendas, often at war with each other. While it's very, very rare for one to actually travel though time, their ability to teleport, move faster than the human eye, and other effects from localized time distortion are the abilities they possess. They remain hidden for a number of reasons, varying from group to group, but their internal conflicts sometimes spill over into normal society - even with their abilities, their conflicts often end in bloodshed and death.
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The Empires of Old - Before recorded human history there were a number of races on continents we think never existed with abilities and technology beyond what old homo-sapiens could do, and they had their own difficulties - Atlantis, Leng, Mu, Lemuria, Hyperborea, Thule and more all existed...and all vanished. Mu was the only one to impart any information to a group Homo Sapiens - the date of their return, the end of the Third Age. December of 2012 saw that return, and now humanity must deal with their returned neighbors - many who think the planet rightfully belongs to them.
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That's what I have for now. I'm sure as I have more ideas I'll pop them up here, but figured I'd get some thoughts on what I have and then go from there. As always, ask questions, praise or mock, whatever you feel like doing, just let me know what you think. :) BTW, with any setting or semi-setting I make, leave a project of yours you want me to review and I'l make sure to do so.
Another Fragment, this one possibly going in Vernith
The Grasping Forest: Upon first entry, one is struck by the fact that the forest is unlike any other seen before. Though trees do grow here, what dominates the landscape are large, rocky structures covered by spikes. It's only by approaching that their true nature is unveiled, as from the top of the structure emerges tentacles that lash out, attempting to grab the intruder and pull them into the waiting maw atop. The structures are shells of an octopus-like monstrosity that lures prey in with a sweet scent only to constrict them into unconsciousness and then devour them whole. Those who know the secret of the forest are few, since it is rare the person that can escape their grasp. However, one humanoid race has cultivated a relationship with these creatures - so long as they bring fresh meat to the Grasping Ones, the creatures do not eat the humanoids. Some cultures, seeing an advantage in such an obscure and deadly predator, have managed to uproot Grasping Ones and "plant" them as defensive structures, a role that they serve with brutal efficiency - though they do not distinguish between friend and foe, consuming all who wander near with equal hunger.