I recently released a new single-player episode for Quake called Deathmatch Dimension. It contains ten maps of classic Quake goodness, inspired by the deathmatch maps that shipped with the original game. Each map has been lovingly rebuilt from the ground up for intense combat with Quake's familiar bestiary, and can be played as a full episode in sequence, or by starting with the shotgun for an extra challenge.
All maps support single-player, coop and deathmatch, and a standard playthrough is probably around two hours long.
Enjoy!
Quick update: Deathmatch Dimension is now an official mod, so is available from the mod menu in all versions of Quake Remastered, including the console releases. Very happy that it can now be played more easily and by more people.
There is also a Nods to the Mods interview if you are interested to know more about the addon and my history making Quake levels.