I've been thinking about an adventuring game with classes, but... different-than-normal classes.
Mages, for instance, are described as those "who devoted themselves to a god, learning the language of the universe and how to manipulate it". Imagine that the world is like your computer, and you learn Java. Binary/Machine Code is the "pure" God Language, Java is the "language" your branch of magi use (there's C++ wizards about!), it comes with boons and drawbacks, and you're a human.
You can't learn the origin language, but by forming a pact with one of the myriad gods, you gain a power. Consider, say, aligning with Ouroboros. You get alchemy through touch, immunity to any toxin, poison, parasite, sickness, virus, or plague magical or otherwise. You even get the gift of infinite life.
The disadvantages? Well, having to consume an ever-rising number of souls before the end of every "life" (80 Years) is quite taxing. At first, you get to buy the next go-around for fairly cheap, but every lifetime, the number doubles. You also partially lose sense of hearing and smell, although your sense of taste is very acute.
Mages can pledge to multiple gods so long as they are not in contest- gods have moral allegiances as well as personal squabbles. Ouroboros is "True Neutral", meaning that you're able to pair/be friendly with/not be smited by any other gods if you align with them as well. You can't leave once you've made a bond though, unless you do something to really piss the god off- generally they'll revoke their power and curse the hell out of you if not outright smite you.
---
Warriors are much the same way. Warriors and rangers pledge to a god, gods, or group of gods, so long as the gods in question don't hate one another. For instance, aligning with Yggdroth (Working on a pure ton of different gods- this one is an eldritch Norse god) will give you strength as well as a few... physical modifications. Warriors often find that they are granted more power for less than with Mages, however, they are often required to be much more active in pleasing their respective god through whatever it is they want. Some gods don't really want anything- Yggdroth, for instance, simply comes with the curse of exuding a dark aura and becoming feared, making social interaction very difficult. You also need to consume flesh of sentients.
Rangers are close to Warriors, though they are afforded different boons/curses. Slyphemes (combination of Slyph and Hermes) grants the user extreme speed and flight, though their hunger is insatiable and they grow weak if they don't eat once every two hours. They do not require sleep, though they can sleep and generally eat a huge dinner so that they can without waking up starved. They gain the very power of the wind, enhancing their speed. They can move at Mach 5, though generally the self-inflicted harm from such a task makes it stupid to do so (Simulations of the SR-71 Blackbird say it can reach Mach 8.2 before the engines just melt from the heat.), especially considering their weakness to impacts (hollow bones).
---
The world is a collection of cities and towns, roughly 75% of the continent is uncivilized, due to wild beasts. Huge gathering areas for the monsters are often called Strata, and they can penetrate many floors into the ground. At the bottom of any Strata is a Boss Monster, that if slain, causes the entire Strata to fall into ruin and become accessible for humans to colonize.
---
The world exists in a very-far-off future, though this is mostly unexplained (because of how distant: WWIII which brings a nuclear end to 99% of humanity is only told of in a sacred legend, and is considered about as old as we consider the story of Adam and Eve). What is explained is the sheer variety of technology and weaponry- people know how to make and use smartphones, there is a such thing as the internet, although it isn't something the average person owns. Most people are villagers who get education in return for some of their produce, most people lead an agricultural life, although anything productive (cottage industry) is profitable. You'll find blacksmiths next to gun-makers, potion shops next to drugstores. Higher technology tends to cost more money, though it all depends on the materials that went into making the item in question.
---
Each Strata gets more and more thematic on the way down. If a Boss Monster set up shop in an ancient secret underground military facility, you may get robotic and gun-toting monsters on the way down, and metal walls beginning to take over the nature that presides above.
---
You would start in a semi-large city. It has all the shops you could possibly need, many nonessential venues, and plenty of places to travel from there.