It doesn't really matter who it is, does it?
How about a fantasy arena game where you can play as one of many classes and build yourself up throughout the game. Some good starter classes would be swordsman, archer, battle mage, monk, or rogue. Include a really good stat system and several advanced classes you can switch to as you progress. Some good advanced classes might be spearman, axe-wielder, ranger, sniper, summoner, elementalist, enchanter, priest, demon hunter, assassin, thief, death knight, paladin, etc. I know it sounds a little generic, but it can still be cool, right?