4. Destructible buildings
If there's anything that decades of playing videogames have taught us, it's that destroying stuff is fun. Strangely, not many action games enable players to take out their rage on the whole environment; a few will maybe let you knock a hole through a wall, or smash through a flimsy barrier, but we can only think of one that enabled you to take out entire buildings on a whim: Mercenaries.
Above: We could raze Pyongyang to the ground in Mercenaries - why not let us smash a few buildings in GTAIV?
Again, this is one place where GTA shouldn't let its competitors get the upper hand. Some might argue that mass destruction on the city-flattening scale of Mercenaries isn't what GTA is about - but why not? There's obviously a demand for it, and if certain buildings - safehouses and the like - are really that vital to the storyline, then there's no harm in placing them off-limits for demolition.
Granted, we've already leveled a building or two in past GTA games, but their destruction was just part of the plot - wedidn't really bring them down ourselves. But imagine being able to take out your rampage-fueling anger on the city itself by going around demolishing office parks with a bazooka. Even if they just regenerated after a few days of game time, it'd still be one of the most cathartic experiences a game could offer.