Game pass is going to help you discover a lot of stuff you probably would've have played before due to the cost of entry on each individual title. Just browse the library and pick something interesting, don't even look at reviews. Lurk or participate in the Xbox Studios OT and Game Pass OTs for suggestions if you are having choice paralysis.
A good step would be to check out some of the bigger games you missed during the XB1 generation. Have you played both Ori games? If not, do so ASAP. Did you like Gears of War when you had a 360? Then you need to check out both Gears 4 and Gears 5 (Gears 5 is a lot better but I still believe Gears 4 is worth playing through!). Also play Hivebusters, a free DLC for Gears 5 which is a standalone story with different characters. The Gears 5 enhancements on Series X are stunning.
You could check out Forza Horizon 4, though with 5 so close to release maybe you just wait for that. There are other great driving games on game pass too like Dirt 5 and Wreckfest. Flight Simulator just to have your mind blown, even if you're not into sims. Go check out a few landmarks or discovery flights.
Psychonauts 2. Do it.
Any Bethesda game you may have missed in the past generation, most of them also have FPS boost support -- for example Prey is amazing with the FPS boost. Have you ever played Yakuza games? Like a Dragon is simply amazing. Play the Titanfall 2 campaign if you never have. RPG fan? What about some Obsidian stuff? The Pillars of Eternity games are great if you like CRPGs. Outer Worlds is worth a play. Wasteland 3! Jedi Fallen Order if you haven't. Sunset Overdrive. Back 4 Blood is coming in October.
Are you looking forward to Halo Infinite? If so maybe a good time to brush up on the franchise and play the Master Chief collection. The whole thing runs at up to 4k120 and looks great (I personally prefer Halo 1 with original graphics, but like the Halo 2 updates). Halo 4 looks ridiculously good for it being a 360 game. Halo 5 is still worth playing too as a lead up, and it looks very good.
I believe everyone should try Sea of Thieves just to see if it's their thing. It's such a neat game. State of Decay 2 is another.
Then in 2022 you have Redfall, Starfield, and whatever else Microsoft cooks up for game pass. If 2020 and 2021 any indication it's gonna be full of more bangers.
Last but not least, take full advantage of the backwards compatibility program. Play some old favorites with enhancements.