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)
You should look for “game name open source”. That should yield good results. Or just looking at the games available in Linux distributions.
Every game from id until (including) Doom 3 was open sourced. So each one of them has such a project.
Off the top of my head there are projects for:
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.
You can find them at openra.net
They have reimplementation of Command & Conquer, Red Alert and Dune 2000. Excellent work really.