I have for you two similar but different ideas. Both would probably be casual level, sandbox style. I am open to giving these ideas away for somebody more experienced to GM instead or I'll try to run it myself.
Entangled Worlds (working title)
Large group roleplay.
There exists four planes of existence: The Heavens (above), The Underworld (below), The Living Realm, (the front, our realm), and The Other Side (the back, alternate realm). Normally, these realms do not cross. But when the stars are aligned just right, strange things can happen and stranger things can seep through the cracks.
The last time this happened was the Dark Ages, and before that, the Age of Mythology. It was during these times that magic was bountiful, fantastic and horrible creatures roamed free, and people gained mysterious powers.
We humans will be pawns in a great chess game between good and evil, light and dark, as these realms collide, and whoever wins decides the fate of the Living Realm.
Too large and powerful to fully enter our world, many Entitites (Gods, Angels, Demons, and Genies and others) grant powers to Champions on earth and urge them to do their bidding. To some it's a blessing, to some a curse, and others a mystery. Each Being has its own way of manipulating its pawn. Some use threats and punishments, some offer rewards, speak clearly, some do nothing at all.
The Roleplay: You can play as a super-powered human, their guiding Entity, free-roaming creatures/entitites, and/or bystanders. Try to just live your life in the changing world or try to shape it--for better or worse.
Bad Entities will often seek a physical body and will manipulate its people and minions to sacrifice themselves or others to serve as its vessel. Good Entities will try to stop this, and make sure that the Living Realm remains free and prosperous (and maybe worships them).
Magical and monstrous creatures will sneak though into the human world to cause chaos or do Entities' bidding. You will discover and interact with just about any kind of mythological or paranormal thing you can think of. Some are good, some bad, some neutral. It's up to you which side you serve, if any.
Setting: Depression era New York City. Imagine something dark and gritty, like DC'S Gotham.
Another similar but more limited idea:
Celestial Powers: Zodiac Rebirth (working title)
Probably around 12 to 20 roleplayers.
A powerful dark matter and energy wave hits Earth, causing many people to become stronger and faster, more stealthy and agile. But those who experience the benefits also become Corrupted. While they become increasingly powerful and gain superpowers, they also turn into heartless psychopaths. Everybody they touch will become infected with the same rage and soulless black eyes. It turns out that invisible Dark Matter parasites are infecting people and turning them into evil, smart zombies to spread their plague across the galaxy.
The planet's sole hope is the Zodiac Warriors: 12 human super-powered individuals who have bonded with 1 of the 12 Celestial Powers who reside in the stars that form the Constellations.
Since the beginning of the Universe these spirits have inhabited other physical beings to serve as their guides in fighting the Dark Plague. While dormant, they radiate their power and affect those lightyears away, influencing the personalities of potential warriors and causing the Zodiac phenomenon we know today.
The roleplay: you play as 1 of 12 human Zodiac Warriors and their guide (or somebody else can be your guide if you prefer), a Seer, or an important regular human. Warriors will most likely embody the traits of their respective Zodiac Guide, but it is not required. Guides and human Warriors may not always see eye-to-eye.
The Zodiacs are drawn together by an instinctive force, and together must exterminate the Dark Matter plague, if they wish to save the Earth. Zodiacs rmit light energy auras that protect them from infection and their touch and powers can purify those it touches, who are not too far gone.
YOU will encounter many super-powered infected individuals. And they will also find people (Seers) who are resistant and can see the infected and blackmatter creatures, who may or may not help in your journey.
Setting: Modern day New York City
Comments, questions, suggestions?