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'm on a discord server with a Win9x era masochist that has made Streets of SimCity, SimCopter, and SimCity 2000, among other games, work on modern systems. His patchers are here: http://krimsky.net/patchers.html
The Mario 64 PC port is a pretty impressive feat, basically a complete open source reverse-engineering of the N64 game which you can then compile as a native binary (using you own copy of the ROM (legally acquired, of course) for the assets) with QoL improvements such as 60fps, wide-screen, high-resolution, free camera movement, and more. Plenty of different versions out there, I liked Render96ex.
It definitely required a little bit of technical knowledge, though that was on Linux, I believe there's a Windows version which might be a bit more beginner-friendly. There are also other forks of the project which might be simpler to start with.
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.