A game where you play as Death Edge, a variation on the grim reaper, a dragon in human skin who uses swords and various style of magic to take on your opponents and collect souls. You would lose you powers by way of something done by the main bad guy, Edge's second Spirit, and would then be charged with killing him. You then have to go get your powers back by collecting souls from gigantic beasties, powerful demons, elemental spirits, and insane gods. By the end of it, you would have your full power back and then some, and then take on the final bad guy in an elaborate multi-part boss fight. So a sort of God of war style Hack-and-slash.
It would be an RPG in the sense that you have to level up, but instead of the classic 100 flat levels of power with fixed or random upgrades, you would get 150 levels and a base increase to all stats and a certain number of points to put on any area of your choosing thereafter, so to specialize in, say, speed, power, defense, magic, etc. You would get experience by the soul-collecting gameplay mechanic, but instead of just red or blue souls, you have to collect souls in an entire spectrum of colors for various purposes.
Red: power souls, get enough to increase attack, speed, and defense for a time
Blue: magic souls, replenish your mana
green: healing souls, replenish health
yellow: fortune souls, used as currency in the god realms to buy stuff
white: pure souls, exp gain
black: damned souls, collect enough to use special attacks
multi-color: god souls, gain new powers, boss fights only
silver, gold, and platinum: Holy souls, extremely rare, use to boost power of weapons or armor.
The armor and weaponary would all be customizeable, and all have specific sets of armor that specialize in one particular area more than another. This would set up a mechanic that allows you to choose your fighting style. For example, speed fighters would substitute defense and attack for speed and evasion, so they would be hard to hit, but while they don't do much damage, they make up for it with multiple strikes. All sets except the starting and final armor sets would specialize, meaning the number of armors would have to be gigantic. Weapons would range from throwing knives to gigantic cleavers, each of which effect the way hte player fights. Ex: a claymore or broadsword would do massive damage and be very helpful in crowd fights, but it would be slow and not able to use more elaborate combos. A pair of daggers or butterfly swords, by comparison, would not do much damage, but allow for very intricate combos that allow the player to really exploit their level. By comparison, magic would be fire, water, ice, lightning, earth, and wind magic, each of which having their own unique points that aid in certain situations. Earth would be useful for close quarters crowd combat, where wind or lightning would be more useful for distant enemies.
It would have to be an open world game, but what I have in mind is something along the lines of the map from Just Cause 2 only multiplied by five. Yeah. You read me. It would be freaking HUGE. The reason being you would have five different worlds to explore.
The physical world: towns, cities, mountains, deserts, forests, plains, and the occasional fight with various animals and demons.
The spirit world: gigantic and foggy, not a lot to see, but plenty of elemental spirits with quests and things to buy.
the god realm: home of the gods, plenty of shops and quests here, but also an arena where you can challenge other gods.
The demon realm: plenty of dark scenery complete with a perpetually blood red sky. For exp grinding, which you will need, because if I were in charge of this the final boss would need you to be at least level 125 before you can eve think about taking him on.
the mirror world: place of the final boss, consists of one gigantic palace and the city surrounding it. As the final mission, it would have you breaking through the defense line until you finally break into the throne room to find the main bad guy waiting there, ending in a fantastic three-part fight wherein he uleashes more and more of his power until the final showdown with you and him in full dragon form.
After beating the game, it would unlock a shorter second story wherein you can play as Edge before he became the mighty death god that he is and learn the secrets of his past, which explain how he got to his possition, why he has virtually no emotion, and where Death Shade, the main villain, came into it all.
This is ALL based around a book I am writing, and believe me, it would make on hell of a game if you could get all of it right. Feel free to PM me if you want to ask me anything relating to the subject.