GTA 5's vehicle cannon mod is made by flocraftMods and does exactly what it sounds like. I'm really enjoying the mod that makes your guns fire cars. The ability to make sweeping changes to the world of Los Santos may yet arrive, but for now I'm enjoying small scripts that let you fly, that make all the pedestrians mad, that make your guns fire cars.
That's what the lack of official modding support in Grand Theft Auto V brings back. There was a time, before mod tools were robust and modding came with a chance of career improvement, when mods were frivolous little things.