If going for more fully open, freedoom is a set of two standalone FPS campaigns(freedoom phase one and two) that also supports doom mods by having free texture/sprite replacments that slot into places where a mod is attempting to use original doom textures/sprites.

For most mods it works well enough but sometimes you'll see a frame of custom animation meant for the original doom monsters.

https://freedoom.github.io/

There's also a deathmatch Wad that works without needing the main doom/freedoom wads, freeDM.