The character creation would be one of the key points, you'd settle on a starting costume for your hero/villain career. Then you'd be taken through the tutorial which would present options that lead to being either a hero or villain.
Players would progress through missions that would follow the story of your heroism, the first 1-10 levels being your origin which would define your allies, enemies and the general direction of your career. There would be more difficult missions, near impossible to solo which would require players to form teams with others. These would be optional to the story, but give more experience and better loot.
Loot wouldn't be traditional equipment, more enhancements for your powers. Each ability within a power set would start with one enhancement slot, which players can increase as they level up. Selecting which powers to increase the slot numbers of, so any powers you don't use within that set don't waste your potential. Enhancements would be set based on class and ability type, so a ranged damaging ability would have different enhancements available compared to a tanking shield.
Player vs Players
PvP for Heroes and Villains would be set to specific maps/zones. Players on the same side, would be able to visit an arena to test their abilities against others via challenges or direct combat. Allowing you to compete with yours friends and challenge your rivals.
The general story for both factions is still to be decided and may influence what the prestige classes would be(unlocked by achieving the maximum level), I was thinking something along the lines of a super powered team that has existed for some years before the players started playing timeline wise. They're professionals in the business, saved the world more times than they can count. However with the new super villain teams emerging from an influx of super powered beings - they're unable to keep up and need assistance.