So, there are projects like these;
https://github.com/ihhub/fheroes2
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.
any and all kind of project keeping an older game alive are welcome, thank you