A massive, sprawling world - deserted in areas, vibrantly populated in others. Cities emerge from this diverse landscape, non-existant in the obvious places, and tucked away in the unseen crevices of the Earth. You, a mercenary on occasion, are venturing across the land, stopping at brothels, enjoying a drink, taking a job, and moving on. Your life is a representation of the state of this world - twisted, roughened, obsolete. In this place of harrowing sins, and devious plots, you are but a puppet. This visage you live under, the traveling adventurer, is but a cloak over your true intent, what you are drawn to by an unyielding, unexplainable force, you reckon is simply your sub-conscious. There are no rules, no laws - your only guiding tenant is to fulfill this immaculate task: find, hunt, and kill the remaining colossi. You are not alone in this hunt - others are unnaturally drawn to complete this task, as if there is a great reward, bestowed upon the victor of the immense, some say impossible task, by an overseeing, ancient, and very powerful creature, be it man, god, or simply a means for these men to put their mind at ease. Your resources - whatever you can find, steal, or acquire. Your strengths - whatever doesn't make you a target to the other hunters. Your possibilities - endless, bound only by your passion to hunt the colossi until you slay the very last one. Work, hunt, hide, run - whatever you do, you will always be forever drawn closer to the colossi, and closer towards your destiny.
These beasts, these colossi, varying in form, in all conceivable aspects, are not an easy thing to hunt. As a mercenary, you have a moderate talent with using simple bladed and blunt weapons, but that is by no means the extent of your arsenal, and definitely not the extent of the other hunters. Harness the ranged arts, be it bow or one of the few rifles you can find in this crippled world. Conjure the spirits and extrapolate powers from the archaic skills of magic, an unpredictable, yet quite deadly tool. Resort to hand-to-hand, or simply avoid combat all together by joining forces with your fellow companions. You have no rules of war, nothing is off-limits in combat - kill the colossi, gather what you can, and move onto the next challenge. You cannot expect these massive, hulking monsters to be in anyway related; they could take to the sky, or strike from beneath the surface - having the possibility of imminent death keeps you on your feet, and makes combat essential to master. You are not alone in this world, even with your thoughts; insanity is not an uncommon thing on this path. Venture far, and venture strong, but do not forget the stretch of reality.
Traveling merchants prey upon the necessity of the world; knowing of the increasing numbers of hunters, you can expect to find isolated merchants selling short knives, magical tomes, flintlock rifles, or even dried foods. While you know not of the origin of your task, why you must complete it, or anything about these strange merchants, you know that you mustn't harm them. The repercussions are unknown - nothing may happen - but with such great risk, would you risk it for a free gun? Bandits sure would, and they are plentiful in this land. Maybe by protecting a particular merchant, you could get a free item; in this unpredictable, chaotic world, you can't be sure of anything.
This world I have been speaking of, this world of danger and reward, is not entirely corrupt. Many cities still remain locked off to the rest of the world, staying safe, or hiding secrets; they have many farms, workers, and citizens, providing an enticing opportunity for those slight of hand, or heavy fisted, but may not be what they seem. Enter at your own risk, through sewers, hidden mountain passes, or simply bust their walls down; what looks like an innocent child in the streets may very well be the bait of a colossi, having ruled over the city for years. Too good to be true, or inconceivably rewarding, these fortresses may very well be the only way to claim your place as victor.
An ever-evolving economy, and capricious atmosphere, the world is vibrant and alive, even in the seemingly desolate and remote areas. Your challenge is fought among thousands of other players, all vying for that title, that completion. To actually win the game is seemingly impossible. More colossi are added randomly, so coordinated attacks, and strategic alliances with the people of this world are necessary if you ever hope to win. By progressing in the challenge - killing more colossi - your level, and skills, will progress. By using your skills, your proficiency in them will increase, and with it, the yield of your actions; more damage from attacks, quicker draws, more money from jobs - more actions and abilities will be available both from this progression, or for purchase and acquisition in the form of training or books. A skill system based on the 3 main forms of combat, you progress into each sector as you please, and ultimately get a specialized class from your choices, karmic retribution, and physical level of strength. Being completely open, while traveling in the world, you may encounter players many levels above you. However, despite PvP in-game, you may not be attacked players more than 10 levels above you, within the first 25 square miles of the world. Anything outside of that, there are no rules stopping you from being harassed, but it is expected that you would have progressed to a reasonable level before entering such dangerous and advanced territory.
As you hunt, you may want to, or need to, gather money, or other resources to sustain. Jobs, or quests, are available randomly, and for dynamic amounts of time. Unlike many other games, each quest that isn't a main story quest can only be completed once, and upon completion, will go back into the 'pot' to be randomly assigned to another NPC. They are first-come, first-serve, and entirely random in both reward and difficulty. There is no telling how difficult a quest really is, aside from what the quest-giver tells you. You could end up fighting a high-level colossi on an errand to deliver a package - with randomly chosen locations, rewards, and quest items, you are going to have to be lucky to survive if you aren't fully prepared.
To understand why you are killing these colossi, you must kill more and more of them, and progress further out into the world. Some colossi will speak riddles, others will be blatant - they are intelligent, and wise; they may even trick you, or lie to you. The ruins, or what seem to be, may contain hints towards the story, but every 10 colossi or so, you are bound to uncover a sliver of the story, that may never be complete. The story may be revealed so slowly, depending on the amount of players, that forums would be left to speculate, practically forced to play more to learn, and get a chance at killing the last of the colossi. Players may never know the story, but once someone wins, a full book would be published, detailing the history, events, and reasons for your hunt.
Seeing how difficult completion would be, it is possible that nobody could ever kill the last colossi. Upon completion, however, if you are the lucky hunter, the hardened soldier, and the master of colossi, then something... unique would happen. For every 100 players, the "pot" increased by $10.00 USD. If you win the game, then you get this pot, in real life. You may complete the game within days, and only get a few hundred dollars, or finish it years after launch, and get hundreds of thousands of dollars. It is yours - your money. The reality of this goal, being both driven by the will to uncover more story, and their eye on the pot, will cause players to fight viciously to win, making a live representation of society, and humanity's inherent greediness.
TL;DR Shadow of the Colossus, Fallout, Dues Ex, The Elder Scrolls, Wendigo, Minecraft, EVE, WoW, Pokemon, Deponia, and Minesweeper combined into the ultimate open-world MMORPG based on the progression of the entire user-base, with an actual monetary reward that increases based on difficulty and users.