Mine is a huge sandbox RPG, with a livid and dynamic 3d world like the Elder Scrolls worlds but even bigger and more interactive. A massive amount of side quests, varying from small ones to huge ones, filled with secrets both menial and impressive. Where all NPC live out there lives, go from place to place, procreate, with large monsters causing deaths and attacking towns that you can save, or not, where things don't respawn, where there are more than one main quest to save, or destroy, the world, and filled with lore, books, and epic storylines, all in a steampunk vs magic world. And there has to be a decent co-op option so you can play with up to a few friends, where your character creation is huge, but also scales, where you will obviously have a better chance defeating a monster at a higher level but am not completely useless at a low level. Where you can create contraptions using parts, or weave spells and train them to your liking. But a world in which nothing is overpowered and death is permanent. Where there are no quest markers but just vague rumors or information you can gather on locations and items and have to find yourself. With near unbeatable puzzles and secrets that can not be seen by the casual eye.
A game you can technically finish after much much time, but then can replay it again and again and the world is never the same depending on your actions.
A game where you can play anything from a psychopath to an angelic passive all-loving super good person that never kills or slays anything, and still be able to beat the quests.
Kind of like a mix of many current games and a little bit more.