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)
Also one of the original goals of GOG was to revive good old games and make them run on modern systems out of the box. Often using one of the open source engines mentioned above. Plus DOSBox.