Patient Gamers

If you found HL1 dated, there's a good remake in source: https://store.steampowered.com/app/362890/Black_Mesa/

Even as a purist (HL1 is my favorite FPS period), I love how Black Mesa handles the source material. It's on sale for $5 right now too. Besides Valve's other games, check out F.E.A.R (2005), Doom series and the Bungie Halo games.

I've been getting into boomer shooters and some more "retro" styled games.

Selaco and Hedon: Bloodrite are imo more immersive sim than boomer shooters, but they can still fit that description as well.

Golden eye 007 for the N64 was pretty revolutionary. As was perfect dark and perfect dark 0, if you don't mind going back to half-life 1 times those are worth a playthrough.

Deus Ex and both System Shocks should be on everyone's lists. I don't really "like" any first-person games (going back to Akalabeth), and I enjoyed both of those games. If you like that style of game, you might also want to try out Thief: The Dark Project.

But I think what comes after HL, given everything, is just Portal. You said "replay" about them, so I guess you already have. So maybe, Narbacular Drop would be the next best thing - it's the game that the Portal devs made before it was Portal. Maybe you'd want to look at The Stanley Parable, too.

There's also Warhammer 40,000: Fire Warrior that's more FPS-y. I don't think you must know anything about 40K to play it, but I couldn't tell you, because I already was into it when I played. Also, the two Star Trek Voyager: Elite Force games, built on expanded versions of the Quake 3 engine.

For historical perspective developed in parallel with Half-Life 2, Medal of Honor: Allied Assault is worth a look. With Call of Duty being the revenue juggernaut it is now, I think we sometimes forget how it started. Allied Assault is a landmark game that's an ancestor in CoD's lineage.

On the other end of the timeline, the founders of the studio that developed the original Call of Duty went on to make Titanfall. Titanfall 2 has an excellent single-player campaign that holds up very well.

The new System Shock remake is very cool and quite underappreciated. The original is far more dated than HL1, IMO. System Shock 2 is playable, though. You could also give the Thief games a whirl, if you played Dishonored. I think along that trajectory you could also go revisit the Hitman games. Hitman 1 and Thief are two takes on the same idea that were happening more or less at the same time. I was more of a Hitman guy at the time, and I think that first game, clunky as it is to control, was mind-blowing.

Are you looking for FPSs specifically? Blizzard is threatening to delist Warcraft 1 and 2 from GOG, so it's a great time to revisit those. 2 holds up.

If you're only looking into FPSs, I'd revisit Quake 2. It's the missing link between HL1 and the arcadey Doom-style early FPSs, and if you have the hardware, the path-traced remake looks really nice.

A few hot takes here: 1) I know I'm in the minority, but I love HL1 and could never get through HL2. The vehicle levels suck, as do the more open areas. The more contained original with its interconnected "Aliens meets Die Hard" setup was so groundbreaking, HL2 is meandering and slow by comparison.

2. I know what you mean about the gravity gun, but also, the Portal devs weren't there for that, but also the puzzle design in Portal is more Valve-y than the stuff in the original Narbacular Drop. I don't know where that lands in reality.

3. I hate the name "immersive sim". What is being "simmed"? Why is it immerisve? Isn't Halo immersive? I was immersed AF. And it's simming at least as much stuff as Dishonored, I assure you. It's such a dumb name, just words mashed together. Ditto for "character action game". Unless your action game features exclusively rocks, it's "character action", that means nothing.