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)
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).
(on reread I see it was gzdoom, oh well)