This is Carmack on Quake V:
"Nothing is scheduled here, people are not building this," Carmack said. "We went from the Quake 2 and the Quake 4 Strogg universe. We are at least tossing around the possibilities of going back to the bizarre, mixed up Cthulhu-ish Quake 1 world and rebooting that direction.
"We think that would be a more interesting direction than doing more Strogg stuff after Quake 4.
We certainly have strong factions internally that want to go do this."
Accidentally this is what I would vote for as well. My most favourite was Quake 1, because it has this unique mix of dark fantasy cover and atmosphere of medival times with sort of cyber mutation in it. This made it stand somewhere in completely different universe. Not military future, not ancient past. Simply unique.
The only thing needed is to save up enough to pay Trent Reznor to come back with his sound design and ambient music once again. His Ghosts I-IV actually recall that Quake 1 spirit sometimes.