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.