Projects that in one way or another aim to modernize a classic, or try make it more playable on modern machine, or just try to expand on things (like gzdoom does)
what other projects are out there in a similar vein?
I find this a little difficult to search for, even with specific terms (i tried finding myself)
Dunno if this is exactly the kind of thing you have in mind, but a person known as 'Yuvi' has been singlehandedly maintaining publicly available servers for the original Demon's Souls for about 4 years (the official servers shut down about 5 years ago).
Thanks to Yuvi, you can still play the original game with all online functionality, regardless of whether you're playing on PS3 or RPCS3.
I was really into this at one point I'm glad it's still about. Played plenty of x3 style games since and none have really been as fun as freelancer was.
There's OpenGOAL which makes the Jak and Daxter games run on modern PCs natively. AFAIK only the first game works flawlessly so far. They're such great games, I'm glad someone is going for it.
Someone also ported Mario64 to PC a while ago. You need to jump through some hoops to get it yourself, or sail the high seas to find an exe file that's already compiled.
Freespace 2 Open, the games source was released in the early 2000's under a ... dubious licence that may or may not be legal to work with but no one really knows and it's been more than 20 years so eh
Freespace 2 far and away my favourite space game, nothing else comes close to the tension of war that game setup so it's great that you don't have to run a .exe from the 90s to play the game. lots of mods for it now too thanks to this project
I see you've mentioned Zdoom, I"ve sank many hours into Brutal Doom (Extermination Day), Blade of Agony and Golden Doom. Found the Hakros Launcher recently which opened some doors (though not the most up to date, still a decent gateway).