Since GoG supports mods now I think it would be cool to add False Dawn mod to Quake 4.
https://www.moddb.com/mods/false-dawn
It is not as long as Doom 3 Phobos, but holds same high quality.
It offers one giant level to explore with 3 different objectives that need to be completed.
You can choose with one will be first and it will increase difficulty of other ones.