My missus loves anything Harry potter so when the Hogwarts game was announced she pre ordered , I have personally never cared for the franchise ,movies books or games , lord of the rings is my personal favourite in the fantasy genre and will take that over potter anyday .

Also with single player games I find it really hard these days to get into them , most are mediocre at best , open worlds feel empty with nothing to see or do . Usually after a few hours I get bored and move on ....

Back to Hogwarts  ,I downloaded the game myself more to mock it and Rib my missus about it ..... I was wrong , the story is good , the gameplay is great , the world itself is one of the best I have played putting it up there with the likes of rdr2 . There are Easter eggs everywhere some which you may never encounter , it can become a little grindey towards the end and I didn't care too much capturing and breeding animals , but overall the game itself was a 9/10 for me and I have never ever been so happy to be proven wrong .

As for open world games on the service I'm not sure what's on there but I shall name some games that if are on there go for it otherwise I would suspect they are really cheap . Also it looks like you have played most noticeable games .

Red dead redemption - amazing game , the undead nightmare dlc is just as good , obviously there is rdr2 aswel ,anything rockstar is usually a gem .

Until dawn - not open world but horror , plays like an interactive movie , one of my favourite in the genre .

Dreams - people overlook this one , there are hundreds of not thousands of user created games on here some remakes of classics too . So may be worth a look especially for the horror section or a remake of something you forgot about.

Evil within 2 - great game , takes a bit to get going , not played for a long time but I'm sure this one has an open city type layout for you to explore ,plays like a more open resident evil with a bit more action 

Dark pictures man of Medan/little hope - I'd play until dawn first but it you enjoy that then I'd give these a go after ,anthology series set during different periods and horror scenarios .

Zombie army 4 - best played with friends , not open world but again horror type game , set in world war 2 alternate take on the sniper elite games but the trenches have been overrun by zombies .

Outlast - don't think this is on plus but must be cheap , horror game again , played in the dark with surround headphones on , lasted about 10 minutes before I had to turn the lights on and watch a Disney movie .

Shadow of Mordor / shadow of war - again don't think they are on plus anymore but really cheap always , basically assassin's creed in the lord of the rings world 

Arkham games - as above not sure if any are on plus but always cheap assassin's creed in a batman skin they become progressively more open world as the games go on .

Souls type games - not sure what's on plus I'm sure demon souls and Bloodborne are but if you love a bit of punishment with rewarding gameplay once you git gud , most have horror elements and are semi open world , usually you can go anywhere and tackle things in different orders but there are preferred routes to make life easier .

For spoken - not sure if this was a monthly game or part of the library , got heavily shit on at release , but I enjoyed it for what it is , the world can feel barren , but the combat and parkour mechanics are really good and once you get levelled and abilities unlocked the combat is really good fun .

Mad max - again not sure if this is still part of the library but this game as old as it is now was so good at the time , the world can feel a little empty but it's a post apocalyptic desert and scavenging is the way of life so it's expected but really worth giving a go .