If your looking for time sink RPG's on gamepass
the obvious choices are Skyrim and Fallout 4. Both are massive and you can download mods to alter the game,change graphics ect.
Other recommendations are The Mass Effect Trilogy and Middle Earth: Shadow Of War.
Also Unisoft have games on there now so Watchdogs 2 and Assassin's Creed Origins.
There's a couple of excellent JRPG'S as well.
Dragon Quest 11 and Octopath Traveler .
I would also recommend The Yakuza Series. All 7 games are on gamepass. All are great games but can be batshit crazy but the stories in each game are excellent.
All the games I mentioned you can easily sink 50 hours plus into each game.
The really big news is that The Persona series is coming to gamepass in October starting with Persona 4 Golden and Persona 5
Persona 4 is one of the best games I've played and both 4 and Persona 5 you get easily clock over 100 hours into them