The superficial glance of the world is one of the faux medieval fantasy many are used to. There are orcs, halflings, etc. An important thing is that there is no magic. There's a bit of cultural superstition, but otherwise it's distinctly naturalist (different from scientific). Low magic to the point of being an atheist's historical fanfiction, were it not for the owlbears and girallons.
This is only the superficial level. Our PCs are going to be mages of one flavour or another, members of an extremely secretive order of spellcasters. The purpose of the party, and the council as a whole, is to suppress all knowledge of the existence of magic from muggles.
The reason for the secrecy is the Other Side. Simply knowing about magic weakens the border between the Material and the Other Side, permitting hostile entities from the Other Side to reach through and attack/invade. Knowing more about magic weakens it further, but the damage to the border is one of diminish returns, and sufficient knowledge will stop doing any damage. The best analogy is to describe the border as a spider web; knowing about magic is like an open palm pressing against the web, stretching and weakening the border. Not enough to permit anyone or anything through, but a large number of people knowing creates a large number of hands pressing together, which can tear open a huge hole. Learning more about magic keeps the total force unchanged, but the palm becomes a fist, increasing the pressure on an area and chances of tearing open a hole to the Other Side. Mages have tightened their fist to a fine enough point to go between the holes in the web, and so cause no damage.
The mages have learned that extradimensional spaces are far enough removed such that knowledge doesn't cause any damage, so they hold council meetings and train new members in mage mansions and the like.
The hostile force of the Other Side? Elementals. From the standard elementals, to golems (after inhabiting local statuary), to elemental outsider elite forces, to the genie overlords commanding the destruction of the Material plane. Magic as a whole is based on elementalism, making every available class some flavour of the elemental wheel.
So as to break from the well trod ground of Ancient Greece's four/five model, I'm going with something closer to the Seven Chakra elements.
- Earth - Includes fear and protection
- Water - Minions and domination
- Fire - Most combat focused, improves sight and speed
- Air - Healing
- Life - Telepathy, electricity
- Death - Illusions, non-undead necromancy
- Time/Space - Buffs, time/space magic