In Serious Sam 2 there was a single-player map, where the ground wasn't flat but a cylinder-like 'pipe'. You could move on the surface of this cylinder only, the gravitation center was inside the pipe, as a looong line. So if you made a jump, you fell back on the cylinder, but when strafing around, it made you feel as you were standind in one place but the enemies and buildings on the cylinder started rolling up and down from left to right or vica versa (imagine as the sun rises then 'rests'). I don't know if it can be done with the Quake engine, however, I think it would add an interesting gameplay.
Buildings can be placed on the cylinder, but also - if you can make a huge pipe around the 'grund-cylinder' (which makes the map 'closed') - there can be built too, Aliens could use their wall-walking ability (because of gravitation Humans could only walk on the main cylinder), build eggs on the 'ceiling' and it would make jetpacks useful.
And if you add walls and doors to cut off pieces from the room between the cylinder and the ensphering 'pipe' (ceiling), you make a no-gravitation like feeling as if it was in space station or something.
I know these all sound crazy, so feedback required
Forward positions are indeed a good idea, so teams won't just camp. It would make the game a little bit like 'Warhammer 40k: Dawn of War' where players have to dominate, control and defend (King of the Hill like) key locations. So, they ought to attack and expand to control as many of these forward bases/fixed turrets/buildings/locations as they can to gain its benefits.