My ideal game would be an "FPS" but instead of guns and other weaponry you use spells/magic. The game would let players choose a set of spells and fight with those in a modern setting. For example a map could be an office building and instead of light machine guns spraying at the guys with shotguns, it would be s guy throwing fireballs while his opponent is lobbing spears of ice with a team coming from the side to flank with an arc of lightning.
A huge focus would be put on making each spell set have its own identity while still granting the utility that others have so that the game is balanced but allows players to play in a style that suits them. So while each spell may have a way to set a damaging trap, Ice will impale them with spikes, Fire would ignite the area, and Lightning would chain between close players.
While this could easily just be another "FPS" style clone, I feel that the mix of scenery and weaponry could make the game stand out.