I honestly have no idea what they could possibly be adding that takes a year to develop. I think when they drop, we're all gonna be thinking "they took a year for THIS?"
I dig what you are saying but most of us have no idea what time is needed to create all new content for a game that was originally made more than 3 years ago. A lot of people on here don't seem to realise ALL the DLC would've been made during original development of the game then they decided what stuff would be DLC. I am hoping they are making heists from the ground up, probably would of needed to bring developers/coders back onto the GTAV project that have moved onto other projects and what not. Total speculation of course.