What if Arothe developed from the ground up?  In the post-Cataclysm era, it represents something typical of the post-apocalyptic genre.  This world is in ruins and everything is lost.  The new world must adapt while trying to salvage the secrets of the old world.  All of these are common place tropes from the end of the world story.  However, Arothe’s old world isn’t a single one, but many!  Each lost world helps to contribute the fantastic history and development of this bizarre new world.  Things start as a mix of a Bronze through Dark Age.  Some cultures directly reflect Earthy ones, while others are a unique blend or independent creation.  From there, the desire to expand through exploration and industry hits the world, not too different from the age of adventure and exploration itself!  This coincides with a cultural renaissance, not too unlike the movements in pre-Crusades Middle East and 1500s Italy.  You have pirates, oceanic expeditions, the start of global communication, and more.  From here, a colony era and global trade era begins to become established as a sort of less brutal/imperialistic look at the 1600s – late 1800s world, especially comparing colonial/frontier United States and post-industrial Eastern Hemisphere.  Modernity meets magic is what equates to a mix of turn of the century through the 1960s.  This is an era of constant war and turbulence.  Entire nations are wiped out from the arcane and atom alike!  Following the Great War, the world develops in a bizarre new direction.  New advances in magicks and sciences lead to exciting and frightening new discovers… some that should not be.  This mirrors an anachronistic mix of the mid/late 1960s into somewhere before now.  From there, Arothe begins to embrace a new future!  While the Vriorax Corporation has made some success encouraging technological development in an otherwise “barbaric” world for centuries, it becomes king here.  Megacorporations replace traditional government, arcane and super-science based technology flourishes, and the world drastically changes for better or worse.  Things develop further in both ways as Arothe’s sciences have led to unimaginable breakthroughs including hyper-advanced space travel and devastating weaponry capable of wiping out entire planets, while causing constant turmoil in and around Arothe!  This intense tension attracts several paranoid alien races.  Being a cautious world itself, Arothe naturally strikes back and insights galactic and civil warfare.  This leads to a grim dark future for Arothe, greater than the “Great War” itself!  But, since this is only the speculation of mad scientists and farfetched mediums, it is likely that alien contact could go in a much more optimistic direction.  Only time will tell!

Now, most likely, I’d start in a “modern setting”, the post-war world or the cyberpunk world.  In a setting where science was left to explore and develop with only a delicate observation to prevent too much depravity, in conjunction with the awareness of “arcane science”, either setting would promote travel through time, albeit at a very restricted allowance.  As time travel is regarded as extremely dangerous, a guild know as The Temporal Agency (post-human beings who make sure the time streams don’t break from severe paradoxes) are ever vigilant.  Recently, several threats have been appearing throughout space and time that are not meant to be, raising much fear and anxiety among the agency.  Not to mention, several divergent timelines have been created from recently executed Agents who have created a number of harmful paradoxes.  These timelines seek to collapse the true one.

Now, in this module, would time travel be central to the plot?  Maybe, maybe not.  It’d likely be used in several plot hooks to link or expand adventure ideas if anything.  I wouldn’t want it becoming the focus either, as you have tons of adventure in the hub world and beyond as it is.  So, would I keep this?  Perhaps!  As for my barrier idea, this seems redundant with worlds outside of the hub world, as I want those dimensions to have their own sets of rules to contrast or accommodate characters.

Space Opera - Fantastic and vibrant life fills the cosmos beyond the planet, either seeking to protect it or destroy it.  Arothe is a central beacon for other world diplomats both extraterrestrial and extraplanar.  Within the main universe and the dimensional rings surrounding, political conflict is almost always high.  Much of the galaxy and beyond is rife with epic adventure, conflict, and drama.

Gothic Horror - There's a dark side to magic and science.  Those who exploit either for selfish ends will corrupt themselves into a disgusting distortion of their former selves.  As a result, many evil factions have arisen from this corruption.  Whether they be mad cyborgs, technophobic beastfolk, the nihilistic elderkind, or many other horrors that be.  While the daytime is full of adventure and thrill, the nighttime is filled with terror and fear.  Dread things lurk in the shadows, waiting for fools to stumble upon them.  Industrialization is to blame for the wrath of nature, nature is to blame for humanity's darkest thoughts. As such, the shadows sometimes dwell deep within our own minds! In fact, there's a good chance YOU'RE the reason for the nightmares that be.

Cosmic Horror - Somewhere lurking in the distance from the hub world and other dimensions is an space between realities.  It should be vacant emptiness, but it isn't.  It's something much world.  The realm beyond the wall of sleep is a place that should not be.  Its existance isn't just a paradox, it's a complete aberration.  Every thing the mind is not meant to handle dwells in the realm.  Some fear the wall is slowly crumbling.

Dystopia – Many communities splintered off from the remains that trash this new world. However, many had a turn for the worse due to their many flaws. Paranoid, isolationist territories became fascist police states, socialized provinces became a communist nightmare. Despite this, many other civilizations have maintained balance in order to prevent hope for the development of society from being crushed by corruption. However, some rare “Utopias” have arisen. Seeing as they're a like minded and small group, they have proven a success. However, these quickly become dystopia for outsiders.

Urban Fantasy – The world of the mystical and fantastic has been modernized by today's standards. Magitech is much more common than mundane technology or pure magic. Many ancient civilizations or realms with a previous “Progress Level” have forced themselves to adapt to a new way of life, or isolate themselves from progress spiraling out of control.

New Weird – Similar to China Mieville and Planescape, there's tons of weird fiction here too! Melding of sci-fi, fantasy, horror to warp worlds both familiar and not. The world has grown far beyond the medieval stereotype and the politics feel equally evolved. The world has science and technology balanced against magic. Technology isn't something to be feared, but rather a neutral party like magic. The world is fantastic without trying to be cliché.

Post-Apocalypse – The world that is now Arothe has been continuously trampled upon and destroyed by cataclysms and dread forces. Invasive powers and species have long since overtaken the planet and realities around this world. Any remnants of the original world are long since killed off. After the end, nothing from the original realm triumphed.