You could always replay Doom 1 & 2 on gog.com; for some reason I still find them entertaining two decades later. Maybe it's the music.
Most FPS games are just different skins slapped on top of the same engine. So it all depends on what kind of skin you want slapped on top of it: shooting at Nazis? Zombies? Space zombies? Nazi zombies? Space Nazi zombies?
Or if you want a different FPS game, have you played the Dishonoured series? I'm finally about to get started on the second one, and I really liked the first. It's not really a shooter as such, more a stealth FPS. One of the aspects that I like is that based on your actions (how many people you kill, how gruesome of a revenge you take) the game actually gets HARDER, with more plague victims wandering about, more alert guards, and swarms of rats that can mulch your legs - and a reasonably different ending for each of the choices: Kill some, kill none, or kill 'em all. Lots of routes through the levels, which makes it rewarding to replay the game.
I got paid a PS4 and Kingdom Hearts 1.5+2.5 "Final Mix XXL Grande Edition With All The Fixins", but after just one trip through KH1 I'm a little burnt out on KH games. It's kinda funny how they added voice acting for Chain of Memories, which takes place between 1 and 2, but Haley Joe Osment's balls dropped in the intervening ten years so it's a major disconnect going from 1 to CoM. Plus, CoM's mechanics SUUUCKED HARD. Just so hard.
But I stand by my assertion that if you changed every instance of the word "heart" with the word "soul" you get a much better dialogue experience in KH.
While I was banned I also played through South Park: Fractured But Whole, and it is WORTH IT if you're a South Park fan. Just freaking hilarious in all aspects. Especially how the kids are mostly concerned about how much money their competing superhero franchises are going to make, because of course that's the most important thing. Timmy as a Professor X psychic was what killed me the most.