The Legend of Zelda: Breath of the Wild
Put about 10 hours in to it so far (I think) and barely scratched the surface, can see this easily being one of those 100 hour plus games (like Fallout 3 and Xenoblade). Openworld is amazing to behold. Standing in the middle of tall grass and watching the wind blow around you may sound very faggy, but it is just awesome. Every thing in game is their to explore, every tree andmountain can be climbed, every tower scaled, plus there's always something to see or do, be it a camp to raid, a giant monster to fight, a puzzle to solve, some wildlife to slaughter for food, a horse to tame, something to climb and throw yourself off etc etc
Some of the systems in the game I really want Red Dead Redemption 2 to adopt, such as the environmental systems where temperature plays a part in how you need to act. Going up a mountain in basic clothes and you'll start freezing to death. Combat it by finding a camp site with a fire, using a torch for warmth, eating hot and spicy food, find warmer clothes...there's several ways to go about it.
Admittedly, I'm starting to feel how I usually do with open world games, and that is overwhelmed. There's so much to see and do that I just don't know what to do, and I'm worried that this will take up months of my gaming time! I left the starting area on Saturday and since then I've been trying to get to a village in the east of the map, and I've only just made it half way because I keep getting distracted. I sat down yesterday to play and decided I'd get to the village in that sitting, but saw a giant creature I wanted to fight and spent the session dying several times and attempting to beat it.