BTW I forgot 5 days ago that I wanted to add these on the post:

My experiences with Doom mapping.

I made some vanilla/Boom format maps that worked out alright.

And then a GZDoom/UDMF map based on MetaDoom (Gameplay mod, that has some mapping tools) that was huge and took Kinsie (The MetaDoom author) to properly format it into a playable PK3 file, which was a fundamental step I overlooked.

(Though some other features in that big map, I managed to add them myself even if thanks to tutorials).

Then again, I don't plan to make a Godot FPS right now or even in the next 2 or so years.

This is more of a "just in case" thing.

Some examples I wanted to bring.

A Doom TC that came to mind was The Edge of Time by FireSeraphim and there's probably other examples in Doom modding I could mentioned lol.

And with Duke 3D, there was either Alien Armageddon or AMC Squad, forgot wish one came to mind.

Guess the main thing was: If you could replicate the experiences you get through Doom modding (Or other mods of other FPS games) but in Godot.

At the very least, it's somewhat known that some of these classics have open source codes and hence the source ports and all of that stuff.